Zoology

ZOOLOGY

THF branch of science to which the name zoology is strictly applicable may be defined as that portion of biology which relates to animals, as distinguished from that portion which is concerned with plants.

The science of biology itself has been placed by Mr Herbert Spencer in the group of concrete sciences, the other groups recognized by that writer being the “abstractconcrete” and the “abstract.” The abstract sciences are logic and mathematics, and treat of the blank forms in which phenomena occur in relation to time, space, and number. The abstract-concrete sciences are mechanics, physics, and chemistry. The title assigned to them is justified by the fact that, whilst their subject-matter is found in a consideration of varied concrete phenomena, they do not aim at the explanation of complex concrete phenomena as such, but at the determination of certain “abstract” quantitative relations and sequences known as the “laws” of mechanics, physics, and chemistry, which never are manifested in a pure form, but always are inferred by observation and experiment upon complex phenomena in which the abstract laws are disguised by their simultaneous interaction. The group of concrete sciences includes astronomy, geology, biology, and sociology. These sciences have for their aim to “explain” the concrete complex phenomena of (a ) the sidereal system, (ό) the earth as a whole, (c) the living matter on the earth’s surface, (d ) human society, by reference to the properties of matter set forth in the generalizations or laws of the abstract-concrete sciences, i.e., of mechanics, physics, and chemistry.

The classification thus sketched exhibits, whatever its practical demerits, the most important fact with regard to biology, namely, that it is the aim or business of those occupied with that branch of science to assign living things, in all their variety of form and activity, to the one set of forces recognized by the physicist and chemist. Just as the astronomer accounts for the heavenly bodies and their movements by the laws of motion and the property of attraction, as the geologist explains the present state of the earth’s crust by the long-continued action of the same forces which at this moment are studied and treated in the form of “laws” by physicists and chemists, so the biologist seeks to explain in all its details the long process of the evolution of the innumerable forms of life now existing, or which have existed in the past, as a necessary outcome, an automatic product, of these same forces.

Science may be defined as the knowledge of causes; and, so long as biology was not a conscious attempt to ascertain the causes of living things, it could not be rightly grouped with other branches of science. For a very long period the two parallel divisions of biology,—botany and zoology,— were actually limited to the accumulation of observations, which were noted, tabulated, and contemplated by the students of these subjects with wonder and delight, but only to a limited extent and in restricted classes of facts with any hope or intention of connecting the phenomena observed with the great nexus of physical sequence or causation. A vague desire to assign the forms and the activities of living things in all their variety to general causes has always been present to thoughtful observers from the earliest times of which we have record, but the earlier attempts in this direction were fantastic in the extreme; and it is the mere truth that, at the time when the phenomena of inorganic nature had been recognized as the outcome of uniform and constant properties capable of analysis and measurement, living things were still left hopelessly out of the domain of explanation, the earlier theories having been rejected and nothing as yet suggested in their place.

The history of zoology as a science is therefore the history of the great biological doctrine of the evolution of living things by the natural selection of varieties in the struggle for existence,—since that doctrine is the one medium whereby all the phenomena of life, whether of form or function, are rendered capable of explanation by the laws of physics and chemistry, and so made the subject-matter of a true science or study of causes. A history of zoology must take account of the growth of those various kinds of information with regard to animal life which have been arrived at in past ages through the labours of a long series of ardent lovers of nature, who in each succeeding period have more and more carefully and accurately tested, proved, arranged, and tabulated their knowledge, until at last the accumulated lore of centuries—almost without the consciousness of its latest heirs and cultivators—took the form of the doctrine of descent and the filiation of the animal series.

History.

There is something almost pathetic in the childish wonder and delight with which mankind in its earlier phases of civilization gathered up and treasured stories of strange animals from distant lands or deep seas, such as are recorded in the Physiologus, in Albertus Magnus, and even at the present day in the popular treatises of Japan and China. That omnivorous universally credulous stage, which may be called the “legendary,” was succeeded by the age of collectors and travellers, when many of the strange stories believed in were actually demonstrated as true by the living or preserved trophies brought to Europe by adventurous navigators. The possibility of verification established verification as a habit; and the collecting of things, instead of the accumulating of reports, developed a new faculty of minute observation. The early collectors of natural curiosities were the founders of zoological science, and to this day the naturalist-traveller and his correlative, the museum curator and systematist, play a most important part in the progress of zoology. Indeed, the historical and present importance of this aspect or branch of zoological science is so great that the name “zoology” has until recently been associated entirely with it, to the exclusion of the study of minute anatomical structure and function which have been distinguished as anatomy and physiology. It is a curious result of the steps of the historical progress of the two divisions of biological science that, whilst the word “botany” has always been understood, and is at the present day understood, as embracing the study, not only of the external forms of plants, their systematic nomenclature and classification, and their geographical distribution, but also the study of their minute structure, their organs of nutrition and reproduction, and the mode of action of the mechanism furnished by those organs, the word “zoology” has been limited to such a knowledge of animals as the travelling sportsman could acquire in making his collections of skins of beasts and birds, of dried insects and molluscs’ shells, and such a knowledge ‘as the museum curator could acquire by the examination and classification of these portable objects. Anatomy and the study of animal mechanism, animal physics, and animal chemistry, all of which form part of a true zoology, have been excluded from the usual definition of the word by the mere accident that the zoologist of the last three centuries has had his museum but has not had his garden of living specimens as the botanist has had; and, whilst the zoologist has thus for a long time been deprived of the means of anatomical and physiological study—only supplied within the past century by the method of preserving animal bodies in alcohol—the demands of medicine for a knowledge of the structure of the human animal have in the meantime brought into existence a separate and special study of human anatomy and physiology.

From these special studies of human structure the knowledge of the anatomy of animals has proceeded, the same investigator who had made himself acquainted with the structure of the human body desiring to compare with the standard given by human anatomy the structures of other animals. Thus comparative anatomy came into existence as a branch of inquiry apart from zoology, and it is only now, in the latter part of the 19th century, that the limitation of the word “zoology” to a knowledge of animals which expressly excludes the consideration of their internal structure has been rejected by the general consent of those concerned in the progress of science; it is now generally recognized that it is mere tautology to speak of zoology and comparative anatomy, and that our museum naturalists must give attention as well to the inside as to the outside of animals.

The anatomy and physiology of plants have never been excluded from the attention of botanists, because in contrast to the earlier zoologists they always were in possession of the whole living plant, raised from seed if need be in a hot-house, instead of having only a dried skin, skeleton, or shell. Consequently the study of vegetable anatomy and physiology has grown up naturally and in a healthy way in strict relation to the rest of botanical knowledge, whilst animal anatomy and physiology have been external to zoology in origin, the product of the medical profession and, as a consequence, subjected to a misleading anthropocentric method.

Whilst we may consider the day as gone by in which zoology could be regarded as connoting solely a special museum knowledge of animals (as twenty-five years ago was still the case), it is interesting to observe by the way the curious usurpation of the word “physiology,” which, from having a wide connotation, indicated by its etymology, —the physiologus of the Middle Ages being nothing more nor less than the naturalist or student of nature,—has in these later days acquired a limitation which it is difficult to justify or explain. Physiology to-day means the study of the physical and chemical properties of the animal or vegetable body, and is even distinguished from the study of structure and strictly confined to the study of function. It would hardly be in place here to discuss at length the steps by which physiology became thus limited, any more than to trace those by which the words “physician” and “physicist” (which both mean one who occupies himself with nature) came to signify respectively a medical practitioner and a student of the laws of mechanics, heat, light, and electricity (but not of chemistry), whilst the word “naturalist” is very usually limited to a lover and student of living things, to the exclusion of the so-called physicist, the chemist, and the astronomer. It is probable that physiology acquired its present significance, viz., the study of the properties and functions of the tissues and organs of living things, by a process of external attraction and spoliation which gradually removed from the original physiologus all his belongings and assigned them to newly named and independently constituted sciences, leaving at last, as a residuum to which the word might still be applied, that medical aspect of life which is concerned with the workings of the living organism regarded as a piece of physico-chemical apparatus.

Whatever may be the history of the word “physiology,” we find zoology, which really started in the 16th century with the awakening of the new spirit of observation and exploration, for a long time running a separate course uninfluenced by the progress of the medical studies of anatomy and physiology. The history of every branch of science involves a recognition of the history, not only of other branches of science, but of the progress of human society in every other relation. The century which destroyed the authority of the church, witnessed the discovery of the New World, and in England produced the writings of Francis Bacon is rightly regarded as the starting-point of the modern knowledge of natural causes or science. The true history of zoology as a science lies within the three last centuries; and, whilst the theories and fables which were current in earlier times in regard to animal life and the various kinds of animals form an important subject of study from the point of view of the history of the development of the human mind, they really have no bearing upon the history of scientific zoology. The great awakening of western Europe in the 16th century led to an active search for knowledge by means of observation and experiment, which found its natural home in the universities. Owing to the connexion of medicine with these seats of learning, it was natural that the study of the structure and functions of the human body and of the animals nearest to man should take root there; the spirit of inquiry which now for the first time became general showed itself in the anatomical schools of the Italian universities of the 16th century, and spread fifty years later to Oxford.

In the 17th century the lovers of the new philosophy, the investigators of nature by means of observation and experiment, banded themselves into academies or societies for mutual support and intercourse. It is difficult to exaggerate the importance of the influence which has been exercised by these associations upon the progress of all branches of science and of zoology especially. The essential importance of academies is to be found, as Laplace, the great French astronomer, has said, “in the philosophic spirit which develops itself in them and spreads itself from them as centres over an entire nation and all relations. The isolated man of science can give himself up to dogmatism without restraint; he hears contradictions only from afar. But in a learned society the enunciation of dogmatic views leads rapidly to their destruction, and the desire of each member to convince the others necessarily leads to the agreement to admit nothing excepting what is the result of observation or of mathematical calculation.”

The first founded of surviving European academies, the Academia Naturae Curiosorum (1651), especially confined itself to the description and illustration of the structure of plants and animals; eleven years later (1662) the Royal Society of London was incorporated by royal charter, having existed without a name or fixed organization for seventeen years previously (from 1645). A little later the Academy of Sciences of Paris was established by Louis XIV. The influence of these great academies of the 17th century on the progress of zoology was precisely to effect that bringing together of the museum-men and the physicians or anatomists which was needed for further development. Whilst the race of collectors and systematizes culminated in the latter part of the 18th century relation to real knowledge, but was part of a peculiar and in itself highly interesting mysticism. A fantastic and elaborate doctrine of symbolism existed which comprised all nature; witchcraft, alchemy, and medicine were its practical expressions. Animals as well as plants were regarded as “simples” and used in medicine, and a knowledge of them was valued from this point of view. Plants were collected and cultivated for medicinal use; hence the physic gardens and the botanist’s advantage.

in Linnaeus, a new type of student made its appearance in such men as John Hunter and other anatomists, who, not satisfied with the superficial observations of the popular “zoologists,” set themselves to work to examine anatomically the whole animal kingdom, and to classify its members by aid of the results of such profound study. From them we pass to the comparative anatomists of the 19th century and the introduction of the microscope as a serious instrument of accurate observation.

The influence of the scientific academies and the spirit in which they worked in the 17th century cannot be better illustrated than by an examination of the early records of the Royal Society of London. The spirit which animated the founders and leaders of that society is clearly indicated in its motto “Nullius in verba.” Marvellous narrations were not permitted at the meetings of the society, but solely demonstrative experiments or the exhibition of actual specimens. Definite rules were laid down by the society for its guidance, designed to ensure the collection of solid facts and the testing of statements embodying novel or remarkable observations. Under the influence of the touchstone of strict inquiry set on foot by the Royal Society, the marvels of witchcraft, sympathetic powders, and other relics of mediaeval superstition disappeared like a mist before the sun, whilst accurate observations and demonstrations of a host of new wonders accumulated, amongst which were numerous contributions to the anatomy of animals, and none perhaps more noteworthy than the observations, made by the aid of microscopes constructed by himself, of Leeuwenhoek, the Dutch naturalist (1683), some of whose instruments were presented by him to the society.

It was not until the 19th century that the microscope, thus early applied by Leeuwenhoek, Malpighi, Hook, and Swammerdam to the study of animal structure, was perfected as an instrument, and accomplished for zoology its final and most important service. The earlier half of the 19th century is remarkable for the rise, growth, and full development of a new current of thought in relation to living things, expressed in the various doctrines of development which were promulgated, whether in relation to the origin of individual animals and plants or in relation to their origin from predecessors in past ages. The perfecting of the microscope led to a full comprehension of the great doctrine of cell-structure and the establishment of the facts—(1) that all organisms are either single corpuscles (so-called cells) of living material (microscopic animalcules, &c.) or are built up of an immense number of such units; (2) that all organisms begin their individual existence as a single unit or corpuscle of living substance, which multiplies by binary fission, the products growing in size and multiplying similarly by binary fission; and (3) that the life of a multicellular organism is the sum of the activities of the corpuscular units of which it consists, and that the processes of life must be studied in and their explanation obtained from an understanding of the chemical and physical changes which go on in each individual corpuscle or unit of living material or protoplasm (cell-theory of Schwann).

On the other hand, the astronomical theories of development of the solar system from a gaseous condition to its present form, put forward by Kant and by Laplace, had impressed men’s minds with the conception of a general movement of spontaneous progress or development in all nature; and, though such ideas were not new but are to be found in some of the ancient Greek philosophers, yet now for the first time they could be considered with a sufficient knowledge and certainty as to the facts, due to the careful observation of the two preceding centuries. The science of geology came into existence, and the whole panorama of successive stages of the earth’s history, each with its distinct population of strange animals and plants, unlike those of the present day and simpler in proportion as they recede into the past, was revealed by Cuvier, Agassiz, and others. The history of the crust of the earth was explained by Lyell as due to a process of slow development, in order to effect which he called in no cataclysmic agencies, no mysterious forces differing from those operating at the present day. Thus he carried on the narrative of orderly development from the point at which it was left by Kant and Laplace,—explaining by reference to the ascertained laws of physics and chemistry the configuration of the earth, its mountains and seas, its igneous and its stratified rocks, just as the astronomers had explained by those same laws the evolution of the sun and planets from diffused gaseous matter of high temperature.

The suggestion that living things must also be included in this great development was obvious. They had been so included by poet-philosophers in past ages; they were so included by many a simple-minded student of nature who, watching the growth of the tree from the seed, formed a true but unverified inference in favour of a general process of growth and development of all things from simpler beginnings. The delay in the establishment of the doctrine of organic evolution was due, not to the ignorant and unobservant, but to the leaders of zoological and botanical science. Knowing as they did the almost endless complexity of organic structures, realizing as they did that man himself with all the mystery of his life and consciousness must be included in any explanation of the origin of living things, they preferred to regard living things as something apart from the rest of nature, specially cared for, specially created by a Divine Being, rather than to indulge in hypotheses which seemed to be beyond all possibility of proof, and were rather of the nature of poets’ dreams than in accordance with the principles of that new philosophy of rigid adherence to fact and demonstration which had hitherto served as the mainsprings of scientific progress. Thus it was that the so-called “Natur-philosophen” of the last decade of the 18th century, and their successors in the first quarter of the 19th, found few adherents among the working zoologists and botanists. Lamarck, Treviranus, Erasmus Darwin, Goethe, and Saint-Hilaire preached to deaf ears, for they advanced the theory that living beings had developed by a slow process of transmutation in successive generations from simpler ancestors, and in the beginning from simplest formless matter, without being able to demonstrate any existing mechanical causes by which such development must necessarily be brought about. They were met in fact by the criticism that possibly such a development had taken place; but, as no one could show as a simple fact of observation that it had taken place, nor as a result of legitimate inference that it must have taken place, it was quite as likely that the past and present species of animals and plants had been separately created or. individually brought into existence by unknown and inscrutable causes, and (it was held) the truly scientific man would refuse to occupy himself with such fancies, whilst ever continuing to concern himself with the observation and record of indisputable facts. The critics did well; for the “Natur-philosophen,” though right in their main conception, were premature.

It was reserved for Charles Darwin, in the year 1859, to place the whole theory of organic evolution on a new footing, and by his discovery of a mechanical cause actually existing and demonstrable by which organic evolution must be brought about to entirely change the attitude in regard to it of even the most rigid exponents of the scientific method. Since its first publication in 1859 the history of Darwin’s theory has been one of continuous and decisive conquest, so that at the present day it is universally accepted as the central, all-embracing doctrine of zoological and botanical science.

Darwin succeeded in establishing the doctrine of organic evolution by the introduction into the web of the zoological and botanical sciences of a new science. The subjectmatter of this new science, or branch of biological science, had been neglected: it did not form part of the studies of the collector and systematist, nor was it a branch of anatomy, nor of the physiology pursued by medical men, nor again was it included in the field of microscopy and the cell-theory. The area of biological knowledge which Darwin was the first to subject to scientific method and to render, as it were, contributory to the great stream formed by the union of the various branches, the outlines of which we have already traced, is that which relates to the breeding of animals and plants, their congenital variations, and the transmission and perpetuation of those variations. This branch of biological science may be called thremmatology (θρέμμa, “a thing bred”). Outside the scientific world an immense mass of observation and experiment had grown up in relation to this subject. From the earliest times the shepherd, the farmer, the horticulturist, and the “fancier” had for practical purposes made themselves acquainted with a number of biological laws, and successfully applied them without exciting more than an occasional notice from the academic students of biology. It is one of Darwin’s great merits to have made use of these observations and to have formulated their results to a large extent as the laws of variation and heredity. As the breeder selects a congenital variation which suits his requirements, and by breeding from the animals (or plants) exhibiting that variation obtains a new breed specially characterized by that variation, so in nature is there a selection amongst all the congenital variations of each generation of a species. This selection depends on the fact that more young are born than the natural provision of food will support. In consequence of this excess of births there is a struggle for existence and a survival of the fittest, and consequently an ever-present necessarily-acting selection, which either maintains accurately the form of the species from generation to generation or leads to its modification in correspondence with changes in the surrounding circumstances which have relation to its fitness for success in the struggle for life.

Darwin’s introduction of thremmatology into the domain of scientific biology was accompanied by a new and special development of a branch of study which had previously been known as teleology, the study of the adaptation of organic structures to the service of the organisms in which they occur. It cannot be said that previously to Darwin there had been any very profound study of teleology, but it had been the delight of a certain type of mind—that of the lovers of nature or naturalists par excellence, as they were sometimes termed—to watch the habits of living animals and plants, and to point out the remarkable ways in which the structure of each variety of organic life was adapted to the special circumstances of life of the variety or species. The astonishing colours and grotesque forms of some animals and plants which the museum zoologists gravely described without comment were shown by these observers of living nature to have their significance in the economy of the organism possessing them; and a general doctrine was recognized, to the effect that no part or structure of an organism is without definite use and adaptation, being designed by the Creator for the benefit of the creature to which it belongs, or else for the benefit, amusement, or instruction of his highest creature—man. Teleology in this form of the doctrine of design was never very deeply rooted amongst scientific anatomists and systematists. It was considered permissible to speculate somewhat vaguely on the subject of the utility of this or that startling variety of structure; but few attempts, though some of great importance, were made to systematically explain by observation and experiment the adaptation of organic structures to particular purposes in the case of the lower animals and plants. Teleology had, however, an important part in the development of what is called physiology, viz., the knowledge of the mechanism, the physical and chemical properties, of the parts of the body of man and the higher animals allied to him. The doctrine of organs and functions—the organ designed so as to execute the function, and the whole system of organs and functions building up a complex mechanism, the complete animal or plant—was teleological in origin (see Physiology), and led to brilliant discoveries in the hands of the physiologists of the last and the preceding century. As applied to lower and more obscure forms of life, teleology presented almost insurmountable difficulties; and consequently, in place of exact experiment and demonstration, the most reckless though ingenious assumptions were made as to the utility of the parts and organs of lower animals, which tended to bring so-called comparative physiology and teleology generally into disrepute. Darwin’s theory had as one of its results the reformation and rehabilitation of teleology. According to that theory, every organ, every part, colour, and peculiarity of an organism, must either be of benefit to that organism itself or have been so to its ancestors: no peculiarity of structure or general conformation, no habit or instinct in any organism, can be supposed to exist for the benefit or amusement of another organism, not even for the delectation of man himself. Necessarily, according to the theory of natural selection, structures either are present because they are selected as useful or because they are still inherited from ancestors to whom they were useful, though no longer useful to the existing representatives of those ancestors.

The conception thus put forward entirely re-founded teleology. Structures previously inexplicable were explained as survivals from a past age, no longer useful though once of value. Every variety of form and colour was urgently and absolutely called upon to produce its title to existence either as an active useful agent or as a survival. Darwin himself spent a large part of the later years of his life in thus extending the new teleology. A beginning only has as yet been made in the new life of that branch of zoological and botanical study.

The old doctrine of types, which was used by the philosophically-minded zoologists (and botanists) of the first half of the century as a ready means of explaining the failures and difficulties of the doctrine of design, fell into its proper place under the new dispensation. The adherence to type, the favourite conception of the transcendental morphologist, was seen to be nothing more than the expression of one of the laws of thremmatology, the persistence of hereditary transmission of ancestral characters, even when they have ceased to be significant or valuable in the struggle for existence, whilst the so-called evidences of design which was supposed to modify the limitations of types assigned to Himself by the Creator were seen to be adaptations due to the selection and intensification by selective breeding of fortuitous congenital variations, which happened to prove more useful than the many thousand other variations which did not survive in the struggle for existence.

Thus not only did Darwin’s theory give a new basis to the study of organic structure, but, whilst rendering the general theory of organic evolution equally acceptable and necessary, it explained the existence of low and simple forms of life as survivals of the earliest ancestry of more highly complex forms, and revealed the classifications of the systematist as unconscious attempts to construct the genealogical tree or pedigree of plants and animals. Finally, it brought the simplest living matter or formless protoplasm before the mental vision as the starting-point whence, by the operation of necessary mechanical causes, the highest forms have been evolved, and it rendered unavoidable the conclusion that this earliest living material was itself evolved by gradual processes, the result also of the known and recognized laws of physics and chemistry, from material which we should call not living. It abolished the conception of life as an entity above and beyond the common properties of matter, and led to the conviction that the marvellous and exceptional qualities of that which we call “living” matter are nothing more nor less than an exceptionally complicated development of those chemical and physical properties which we recognize in a gradually ascending scale of evolution in the carbon compounds, containing nitrogen as well as oxygen, sulphur, and hydrogen as constituent atoms of their enormous molecules. Thus mysticism was finally banished from the domain of biology, and zoology became one of the physical sciences, —the science which seeks to arrange and discuss the phenomena of animal life and form as the outcome of the operation of the laws of physics and chemistry.

Nature and Scope of Zoology.

The brief historical outline above given is sufficient to justify us in rejecting, for the purposes of an adequate appreciation of the history and scope of zoology, that simple division of the science into morphology and physiology which is a favourite one at the present day. No doubt the division is a logical one, based as it is upon the distinction of the study of form and structure in themselves (morphology) from the study of what are the activities and functions of the forms and structures (physiology). Such logical divisions are possible upon a variety of bases, but are not necessarily conducive to the ascertainment and remembrance of the historical progress and present significance of the science to which they are applied. As a matter of convenience and as the outcome of historical events it happens that in the universities of Europe, whilst botany in its entirety is usually represented by one chair, the animal side of biology is represented by a chair of so-called zoology, which is understood as the old-fashioned systematic zoology, a chair of human and comparative anatomy, and a chair of physiology (signifying the mechanics, physics, and chemistry of animals especially in relation to man). Fifty years ago the chairs of anatomy and physiology were united in one. No such distinction of mental activities as that involved in the division of the study of animal life into morphology and physiology has ever really existed: the investigator of animal forms has never entirely ignored the functions of the forms studied by him, and the experimental inquirer into the functions and properties of animal tissues and organs has always taken very careful account of the forms of those tissues and organs.

A more instructive subdivision of the science of animal biology or zoology is one which shall correspond to the separate currents of thought and mental preoccupation which have been historically manifested in western Europe in the gradual evolution of what is to-day the great river of zoological doctrine to which they have all been rendered contributory. Such a subdivision of zoology, whilst it enables us to trace the history of thought, corresponds very closely with the actual varieties of mental attitude exhibited at the present day by the devotees of zoological study, though it must be remembered that the gathering together of all the separate currents by Darwin is certain sooner or later to entail new developments and branchings of the stream.

We accordingly recognize the following five branches of zoological study :—

1. Morphography.— The work of the collector and systematist: exemplified by Linnaeus and his predecessors, by Cuvier, Agassiz, Haeckel.

2. Bionomics.— The lore of the farmer, gardener, sportsman, fancier, and field-naturalist, including thremmatology, or the science of breeding, and the allied teleology, or science of organic adaptations: exemplified by the patriarch Jacob, the poet Virgil, Sprengel, Kirby and Spence, Wallace, and Darwin.

3. Zoo-Dynamics, Zoo-Physics, Zoo-Chemistry. — The pursuit of the learned physician,—anatomy and physiology: exemplified by Harvey, Haller, Hunter, Johann Müller.

4. Plasmology.— The study of the ultimate corpuscles of living matter, their structure, development, and properties, by the aid of the microscope; exemplified by Malpighi, Hook, Schwann, Kowalewsky.

5. Philosophical Zoology.— General conceptions with regard to the relations of living things (especially animals) to the universe, to man, and to the Creator, their origin and significance: exemplified in the writings of the philosophers of classical antiquity, and of Linnaeus, Goethe, Lamarck, Cuvier, Lyell, H. Spencer, and Darwin.

It is true that it is impossible to assign the great names of the present century to a single one of the subdivisions of the science thus recognized. With men of an earlier date such special assignment is possible, and there would be no difficulty about thus separating the minor specialists of modern times. But the fact is that as we approach Darwin’s epoch we find the separate streams more and more freely connected with one another by anastomosing branches; and the men who have left their mark on the progress of science have been precisely those who have been instrumental in bringing about such confluence, and have distinguished themselves by the influence of their discoveries or generalizations upon several lines of work. At last, in Darwin we find a name which might appear in each of our subdivisions,—a zoologist to whose doctrine all are contributory, and by whose labours all are united and reformed.

We shall now briefly sketch the history of these streams of thought, premising that one has (so far as the last three centuries are concerned) but little start of another, and that sooner or later the influence of the progress of one branch makes itself felt in the progress of another.

Morphography.

Under this head we include the systematic exploration and tabulation of the facts involved in the recognition of all the recent and extinct kinds of animals and their distribution in space and time. (1) The museum-makers of old days and their modern representatives the curators and describers of zoological collections, (2) early explorers and modern naturalist-travellers and writers on zoo-geography, and (3) collectors of fossils and palaeontologists are the chief varieties of zoological workers coming under this head. Gradually since the time of Hunter and Cuvier anatomical study has associated itself with the more superficial morphography until to-day no one considers a study of animal form of any value which does not include internal structure, histology, and embryology in its scope.

The real dawn of zoology after the legendary period of the Middle Ages is connected with the name of an Englishman, Wotton, born at Oxford in 1492, who practised as a physician in London and died in 1555. He published a treatise De Differentiis Animalium at Paris in 1552. In many respects Wotton was simply an exponent of Aristotle, whose teaching, with various fanciful additions, constituted the real basis of zoological knowledge throughout the Middle Ages. It was Wotton’s merit that he rejected the legendary and fantastic accretions, and returned to Aristotle and the observation of nature. The most ready means of noting the progress of zoology during the 16th, 17th, and 18th centuries is to compare the classificatory conceptions of successive naturalists with those which are to be found in the works of Aristotle himself. Aristotle did not definitely and in tabular form propound a classification of animals, but from a study of his treatises Historia Animalium, De Generatione Animalium, and De Partibus Animalium the following classification can be arrived at :—

A. ΅Evαιμα, blood-holding animals ( = Vertebrata).

1. Zωoτoκoῠvτα έ v aύτοῖs, viviparous Enaema ( = Mammals, including the Whale).

2. "Oρvιθεs ( = Birds).

3. Τετράποδα ᾔ ᾂποδα ώοτοκοΰντα, four-footed or legless Enaema which lay eggs ( = Reptiles and Amphibia).

4. 'Iχθύϵς ( = Fishes).

B. "Αναιμα, bloodless animals ( = Invertebrata).

1. Μαλάκια, soft-bodied Anaema ( = Cephalopoda).

2. Μαλακόστρακα, soft-shelled Anaema ( = Crustacea).

3. "Εντομα, insected Anaema or Insects (= Arthropoda, exclusive of Crustacea).

4. Όστρακοδέρματα, shell-bearing Anaema (=Echini, Gastropoda, and Lamellibranchia. )

Wotton follows Aristotle in the division of animals into the Enaema and the Anaema, and in fact in the recognition of all the groups above given, adding only one large group to those recognized by Aristotle under the Anaema, namely, the group of Zooρhyta, in which Wotton includes the Holothuriae, Star-Fishes, Medusae, Sea-Anemones, and Sponges. Wotton divides the viviparous quadrupeds into the manytoed, double-hoofed, and single-hoofed. By the introduction of a method of classification which was due to the superficial Pliny,—viz., one depending, not on structure, but on the medium inhabited by an animal, whether earth, air, or water,—Wotton is led to associate Fishes and Whales as aquatic animals. But this is only a momentary lapse, for he broadly distinguishes the two kinds.

Conrad Gesner (1516-1565), who was a physician and held professorial chairs in various Swiss cities, is the most voluminous and instructive of these earliest writers on systematic zoology, and was so highly esteemed that his Historia Animalium was republished a hundred years after his death. His great work appeared in successive parts,— e.g., Vivipara, Ovipara, Aves, Pisces, Serpentes et Scorpio,— and contains descriptions and illustrations of a large number of animal forms with reference to the lands inhabited by them. Gesner’s work, like that of John Johnstone (b. 1603), who was of Scottish descent and studied at St Andrews, and like that of Ulysses Aldrovandi of Bologna (b . 1522), was essentially a compilation, more or less critical, of all such records, pictures, and relations concerning beasts, birds, reptiles, fishes, and monsters as could be gathered together by one reading in the great libraries of Europe, travelling from city to city, and frequenting the company of those who either had themselves passed into distant lands or possessed the letters written and sometimes the specimens brought home by adventurous persons.

The exploration of parts of the New World next brought to hand descriptions and specimens of many novel forms of animal life, and in the latter part of the 16th century and the beginning of the 17th that careful study by “specialists” of the structure and life-history of particular groups of animals was commenced which, directed at first to common and familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. This minuter study had two origins, one in the researches of the medical anatomists, such as Fabricius (1537-1619), Severinus (1580-1656), Harvey (1578-1657), and Tyson (1649-1708), the other in the careful work of the entomologists and first microscopists, such as Malpighi (1628-1694), Swammerdam (1637-1680), and Hook (1635-1702). The commencement of anatomical investigations deserves notice here as influencing the general accuracy and minuteness with which zoological work was prosecuted, but it was not until a late date that their full influence was brought to bear upon systematic zoology by Georges Cuvier (1769- 1832).

The most prominent name between that of Gesner and Linnaeus in the history of systematic zoology is that of John Ray ( q.v.). Though not so extensive as that of Linnaeus, his work is of the highest importance, and rendered the subsequent labours of the Swedish naturalist far easier than they would otherwise have been. A chief merit of Ray is to have limited the term “species” and to have assigned to it the significance which it has until the Darwinian era borne, whereas previously it was loosely and vaguely applied. He also made considerable use of anatomical characters in his definitions of larger groups, and may thus be considered as the father of modern zoology. Associated with Ray in his work, and more especially occupied with the study of the Worms and Mollusca, was Martin Lister (1638-1712), who is celebrated also as the author of the first geological map.

After Ray’s death in London in 1705 the progress of anatomical knowledge, and of the discovery and illustration of new forms of animal life from distant lands, continued with increasing vigour. We note the names of Vallisnieri (1661-1730) and Alexander Monro (1697-1767); the travellers Tournefort (1656-1708) and Shaw (1692-1751); the collectors Rumphius (1637-1706) and Hans Sloane (1660-1753); the entomologist Réaumur (1683-1757); Lhwyd (1703) and Linck (1674-1734), the students of StarFishes; Peyssonel (b. 1694), the investigator of Polyps and the opponent of Marsigli and Réaumur, who held them to be plants; Woodward, the palaeontologist (1665- 1722),—not to speak of others of less importance.

Two years after Ray’s death Carl Linnaeus ( q.v. ) was born. Unlike Jacob Theodore Klein (1685-1759), whose careful treatises on various groups of plants and animals were published during the period between Ray and Linnaeus, the latter had his career marked out for him in a university, that of Upsala, where he was first professor of medicine and subsequently of natural history. His lectures formed a new departure in the academic treatment of zoology and botany, which, in direct continuity from the Middle Ages, had hitherto been subjected to the traditions of the medical profession and regarded as mere branches of “materia medica.” Linnaeus taught zoology and botany as branches of knowledge to be studied for their own intrinsic interest. His great work, the Systema Naturae, ran through twelve editions during his lifetime (1st ed. 1735, 12th 1768). Apart from his special discoveries in the anatomy of plants and animals, and his descriptions of new species, the great merit of Linnaeus was his introduction of a method of enumeration and classification which may be said to have created systematic zoology and botany in their present form, and establishes his name for ever as the great organizer, the man who recognized a great practical want in the use of language and supplied it. Linnaeus adopted Ray’s conception of species, but he made species a practical reality by insisting that every species shall have a double Latin name,—the first half to be the name of the genus common to several species, and the second half to be the specific name. Previously to Linnaeus long many-worded names had been used, sometimes with one additional adjective, sometimes with another, so that no true names were fixed and accepted. Linnaeus by his binomial system made it possible to write and speak with accuracy of any given species of plant or animal. He was, in fact, the Adam of zoological science. He proceeded further to introduce into his enumeration of animals and plants a series of groups, viz., genus, order, class, which he compared to the subdivisions of an army or the subdivisions of a territory, the greater containing several of the less, as follows :— [table]

Linnaeus himself recognized the purely subjective character of his larger groups; for him species were, however, objective: “there are,” he said, “just so many species as in the beginning the Infinite Being created.” It was reserved for a philosophic zoologist of the 19th century (Agassiz, Essay on Classification, 1859) to maintain dogmatically that genus, order, and class were also objective facts capable of precise estimation and valuation. This climax was reached at the very moment when Darwin was publishing the Origin of Species (1859), by which universal opinion has been brought to the position that species, as well as genera, orders, and classes, are the subjective expressions of a vast ramifying pedigree in which the only objective existences are individuals, the apparent species as well as higher groups being marked out, not by any distributive law, but by the purely non-significant operation of human experience, which cannot transcend the results of death and decay.

The classification of Linnaeus (from Syst. Nat., 12th ed., 1766) should be compared with that of Aristotle. It is as follows,—the complete list of Linnaean genera being here reproduced :— [table] [table]

[table]

The characters of the six classes are thus given by Linnaeus :— [table]

Between Linnaeus and Cuvier there are no very great names; but under the stimulus given by the admirable method and system of Linnaeus observation and description of new forms from all parts of the world, both recent and fossil, accumulated. We can only cite the names of Charles Bonnet (1720-1793), the entomologist, who described the reproduction of Aphis ; Banks and Solander, who accompanied Captain Cook on his first voyage (1768-1771); Thomas Pennant (1726-1798), the describer of the English fauna; Peter Simon Pallas (1741-1811), who specially extended the knowledge of the Linnaean Vermes, and under the patronage of the empress Catherine explored Russia and Siberia; De Geer (1720-1778), the entomologist; Lyonnet (1707-1789), the author of the monograph of the anatomy of the caterpillar of Cossus ligniperdus ; Cavolini (1756- 1810), the Neapolitan marine zoologist and forerunner of Della Chiaje (fl. 1828); O. F. Müller (1730-1784), the describer of freshwater Oligoehaeta; Abraham Trembley (1700-1784), the student of Hydra ·, and Ledermüller (1719- 1769), the inventor of the term Infusoria. The effect of the Linnaean system upon the general conceptions of zoologists was no less marked than were its results in the way of stimulating the accumulation of accurately observed details. The notion of a scala naturae, which had since the days of classical antiquity been a part of the general philosophy of nature amongst those who occupied themselves with such conceptions, now took a more definite form in the minds of skilled zoologists. The species of Linnaeus were supposed to represent a series of steps in a scale of ascending complexity, and it was thought possible thus to arrange the animal kingdom in a single series,—the orders within the classes succeeding one another in regular gradation, and the classes succeeding one another in a similar rectilinear progression. Lamarck (q.v.) represents most completely, both by his development theory (to be further mentioned below) and by his scheme of classification, the high-water mark of the popular but fallacious conception of a scala naturae. His classification (1801 to 1812) is as follows :— [table]

The enumeration of orders above given will enable the reader to form some conception of the progress of knowledge relating to the lower forms of life during the fifty years which intervened between Linnaeus and Lamarck. The number of genera recognized by Lamarck is more than ten times as great as that recorded by Linnaeus.

We have mentioned Lamarck before his great contemporary Cuvier because, in spite of his valuable philosophical doctrine of development, he was, as compared with Cuvier and estimated as a systematic zoologist, a mere enlargement and logical outcome of Linnaeus.

The distinctive merit of Cuvier (q.v.) is that he started a new view as to the relationships of animals, which he may be said in a large measure to have demonstrated as true by his own anatomical researches. He opposed the scala naturae theory, and recognized four distinct and divergent branches or embranchemens, as he called them, in each of which he arranged a certain number of the Linnaean classes, or similar classes. The embranchemens were characterized each by a different type of anatomical structure. Cuvier thus laid the foundation of that branching tree-like arrangement of the classes and orders of animals which we now recognize as being the necessary result of attempts to represent what is practically a genealogical tree or pedigree. Apart from this, Cuvier was a keen-sighted and enthusiastic anatomist of great skill and industry. It is astonishing how many good observers it requires to dissect and draw and record over and over again the structure of an animal before an approximately correct account of it is obtained. Cuvier dissected many Molluscs and other animals which had not previously been anatomized; of others he gave more correct accounts than had been given by earlier writers. Skilful as he was, his observations are very frequently erroneous. Great accuracy in work as well as great abundance of production has only distinguished one amongst all the great names of zoology—that of Johann Müller. It certainly did not distinguish Cuvier. Another special distinction of Cuvier is his remarkable work in comparing extinct with recent organisms, his descriptions of the fossil Mammalia of the Paris basin, and his general application of the knowledge of recent animals to the reconstruction of extinct ones, as indicated by fragments only of their skeletons.

It was in 1812 that Cuvier communicated to the Academy of Sciences of Paris his views on the classification of animals. He says—

"Si l'on considère le règne animal d’après les principes que nous venons de poser, en se debarassant des préjugés établis sur les divisions anciennement admises, en n’ayant égard qu’ à l’organisation et à la nature des animaux, et non pas à leur grandeur, à leur utilité, au plus ou moins de connaissance que nous en avons, ni à toutes les autres circonstances accessoires, on trouvera qu’il existe quatre formes principales, quatre plans généraux, si l’on peut s’exprimer ainsi, d’après lesquels tous les animaux semblent avoir été modelés et dont les divisions ultérieures, de quelque titre que les naturalistes les aient décorées, ne sont que des modifications assez légères, fondées sur le développement, ou l’addition de quelques parties qui ne changent rien à l’essence du plan.”

His classification as finally elaborated in Le Règne Animal (Paris, 1829) is as follows :— [table]

The leading idea of Cuvier, his four embranchemens, was confirmed by the Russo-German naturalist Von Baer (1792-1876), who adopted Cuvier’s divisions, speaking of them as the peripheric, the longitudinal, the massive, and the vertebrate types of structure. Von Baer, however, has another place in the history of zoology, being the first and most striking figure in the introduction of embryology into the consideration of the relations of animals to one another.

Cuvier may be regarded as the zoologist by whom anatomy was made the one important guide to the understanding of the relations of animals. But it should be noted that the belief, dating from Malpighi (1670), that there is a relationship to be discovered, and not merely a haphazard congregation of varieties of structure to be classified, had previously gained ground. Cuvier was familiar with the speculations of the “Natur-philosophen,” and with the doctrine of transmutation and filiation by which they endeavoured to account for existing animal forms. The noble aim of F. W. J. Schelling, “das ganze System der Naturlehre von dem Gesetze der Schwere bis zu den Bildungstrieben der Organismus als ein organisches Ganze darzustellen,” which has ultimately been realized through Darwin, was a general one among the scientific men of the year 1800. Lamarck accepted the development theory fully, and pushed his speculations far beyond the realm of fact. The more cautious Cuvier adopted a view of the relationships of animals which, whilst denying genetic connexion as the explanation, recognized an essential identity of structure throughout whole groups of animals. This identity was held to be due to an ultimate law of nature or the Creator’s plan. The tracing out of this identity in diversity, whether regarded as evidence of blood-relationship or as a remarkable display of skill on the part of the Creator in varying the details whilst retaining the essential, became at this period a special pursuit, to which Goethe, the poet, who himself contributed importantly to it, gave the name “morphology.” C. F. Wolff, Goethe, and Oken share the credit of having initiated these views, in regard especially to the structure of flowering plants and the Vertebrate skull. Cuvier’s doctrine of four plans of structure was essentially a morphological one, and so was the single-scale doctrine of Buffon and Lamarck, to which it was opposed. Cuvier’s morphological doctrine received its fullest development in the principle of the “correlation of parts,” which he applied to palaeontological investigation, namely, that every animal is a definite whole, and that no part can be varied without entailing correlated and law-abiding variations in other parts, so that from a fragment it should be possible, had we a full knowledge of the laws of animal structure or morphology, to reconstruct the whole. Here Cuvier was imperfectly formulating, without recognizing the real physical basis of the phenomena, the results of the laws of heredity, which were subsequently investigated and brought to bear on the problems of animal structure by Darwin.

Richard Owen may be regarded as the foremost of Cuvier’s disciples. Owen not only occupied himself with the dissection of rare animals, such as the Pearly Nautilus, Lingula, Limulus, Protopterus, Apteryx, &c., and with the description and reconstruction of extinct Reptiles, Birds, and Mammals,—following the Cuvierian tradition,—but gave precision and currency to the morphological doctrines which had taken their rise in the beginning of the century by the introduction of two terms, “homology” and “analogy,” which were defined so as to express two different kinds of agreement in animal structures, which, owing to the want of such “counters of thought,” had been hitherto continually confused. Analogous structures in any two animals compared were by Owen defined as structures performing similar functions, but not necessarily derived from the modification of one and the same part in the “plan” or “archetype” according to which the two animals compared were supposed to be constructed. Homologous structures were such as, though greatly differing in appearance and detail from one another, and though performing widely different functions, yet were capable of being shown by adequate study of a series of intermediate forms to be derived from one and the same part or organ of the “plan-form” or “archetype.” It is not easy to exaggerate the service rendered by Owen to the study of zoology by the introduction of this apparently small piece of verbal mechanism; it takes place with the classificatory terms of Linnaeus. And, though the conceptions of “archetypal morphology,” to which it had reference, are now abandoned in favour of a genetic morphology, yet we should remember, in estimating the value of this and of other speculations which have given place to new views in the history of science, the words of the great reformer himself. “Erroneous observations are in the highest degree injurious to the progress of science, since they often persist for a long time. But erroneous theories, when they are supported by facts, do little harm, since every one takes a healthy pleasure in proving their falsity” (Darwin). Owen’s definition of analogous structures holds good at the present day. His homologous structures are now spoken of as “homogenetic” structures, the idea of community of representation in an archetype giving place to community of derivation from a single representative structure present in a common ancestor. Darwinian morphology has further rendered necessary the introduction of the terms “homoplasy” and “homoplastic” 1 to express that close agreement in form which may be attained in the course of evolutional changes by organs or parts in two animals which have been subjected to similar moulding conditions of the environment, but have no genetic community of origin, to account for their close similarity in form and structure.

The classification adopted by Owen in his lectures (1855) does not adequately illustrate the progress of zoological knowledge between Cuvier’s death and that date, but, such as it is, it is worth citing here. [table]

The real centre of progress of systematic zoology was no longer in France nor with the disciples of Cuvier in England, but after his death moved to Germany. The wave of morphological speculation, with its outcome of new systems and new theories of classification, which were as numerous as the professors of zoological science, 2 was necessarily succeeded in the true progress of the science by a period of minuter study in which the microscope, the discovery of embryological histories, and the all-important cell-theory came to swell the stream of exact knowledge.

We have already mentioned Von Baer in this connexion, and given a passing reference to Johann Müller (q.v. ), the greatest of all investigators of animal structure in the present century. Müller (1801-1858) was in Germany the successor of Rathke (1793-1860) and of Meckel (1781-1833) as the leader of anatomical investigation; but his true greatness can only be estimated by a consideration of the fact that he was a great teacher not only of human and comparative anatomy and zoology but also of physiology, and that nearly all the most distinguished German zoologists and physiologists of the period 1850 to 1870 were his pupils and acknowledged his leadership. The most striking feature about Johann Müller’s work, apart from the comprehensiveness of his point of view, in which he added to the anatomical and morphological ideas of Cuvier a consideration of physiology, embryology, and microscopic structure, was the extraordinary accuracy, facility, and completeness of his recorded observations. He could do more with a single specimen of a rare animal (e.g., in his memoir on Amphioxus, Berlin, 1844) in the way of making out its complete structure than the ablest of his contemporaries or successors could do with a plethora. His power of rapid and exhaustive observation and of accurate pictorial reproduction was phenomenal. His most important memoirs, besides that just mentioned, are those on the anatomy and classification of Fishes, on the Coecilians, and on the developmental history of the Echinoderms.

See Lankester, “On the Use of the Term Homology in Modem Zoology and the Distinction between Homogenetic and Homoplastic Agreements,” Ann. and Mag. Nat. Hist. 1870.

See Agassiz, Essay on Classification, 1859, for an account of them.

A name which is apt to be forgotten in the period between Cuvier and Darwin, because its possessor occupied an isolated position in England and was not borne up by any great school or university, is that of John Vaughan Thompson, who was an army surgeon, and when past the age of forty, being district medical inspector at Cork (1830), took to the study of marine Invertebrate by the aid of the microscope. Thompson made three great discoveries, which seem to have fallen in his way in the most natural and simple manner, but must be regarded really as the outcome of extraordinary genius. He showed that the organisms like Flustra are not hydroid Polyps, but of a more complex structure resembling Molluscs, and he gave them the name Polyzoa. He discovered the Pentacrinus europaeus, and showed that it was the larval form of the Feather-Star Antedon (Comatula). He upset Cuvier’s retention of the Cirripedes among Mollusca, and his subsequent treatment of them as an isolated class, by showing that they begin life as free-swimming Crustacea identical with the young forms of other Crustacea. Vaughan Thompson is a type of the marine zoologists, such as Dalyell, Michael Sars, P. J. Van Beneden, Claparède, and Allman, who during the present century have approached the study of the lower marine organisms in the same spirit as that in which Trembley and Schäffer in the last century, and Swammerdam in the 17th, gave themselves to the study of the minute freshwater forms of animal life.

It is impossible to enumerate or to give due consideration to all the names in the army of anatomical and embryological students of the middle third of this century whose labours bore fruit in the modification of zoological theories and in the building up of a true classification of animals. Their results are best summed up in the three schemes of classification which follow below—those of Rudolph Leuckart (b. 1823), Henri Milne-Edwards (1800-1884), and T. H. Huxley (b. 1825), all of whom individually contributed very greatly by their special discoveries and researches to the increase of exact knowledge.

Contemporaneous with these were various schemes of classification which were based, not on a consideration of the entire structure of each animal, but on the variations of a single organ, or on the really non-significant fact of the structure of the egg. All such single-fact systems have proved to be useless and in fact departures from the true line of growth of the zoological system which was shaping itself year by year—unknown to those who so shaped it— as a genealogical tree. They were attempts to arrive at a true knowledge of the relationships of animals by “royal roads”; their followers were landed in barren wastes.

R. Leuckart’s classification is as follows :— [table]

The classification given by Henri Milne-Edwards is as follows :— [table] [table]

(The orders of the classes which follow are not given in the work quoted. ) [table]

In England T. H. Huxley adopted in his lectures (1869) a classification which was in many respects similar to both of the foregoing, but embodied improvements of his own. It is as follows :— [table]

We now arrive at the period when the doctrine of organic evolution was established by Darwin, and when naturalists, being convinced by him as they had not been by the transmutationists of fifty years’ earlier date, were compelled to take an entirely new view of the significance of all attempts at framing a “natural” classification.

Many zoologists—prominent among them in Great Britain being Huxley—had been repelled by the airy fancies and assumptions of the “philosophical” morphologists. The efforts of the best minds in zoology had been directed for thirty years or more to ascertaining with increased accuracy and minuteness the structure, microscopic and gross, of all possible forms of animals, and not only of the adult structure but of the steps of development of that structure in the growth of each kind of organism from the egg to maturity. Putting aside fantastic theories, these observers endeavoured to give in their classifications a strictly objective representation of the facts of animal structure and of the structural relationships of animals to one another capable of demonstration. The groups within groups adopted for this purpose were necessarily wanting in symmetry: the whole system presented a strangely irregular character. From time to time efforts were made by those who believed that the Creator must have followed a symmetrical system in his production of animals to force one or other artificial, neatly balanced scheme of classification upon the zoological world. The last of these was that of Louis Agassiz (Essay on Classification, 1859), who, whilst surveying all previous classifications, propounded a scheme of his own, in which, as well as in the criticisms he applies to other systems, the leading notion is that sub-kingdoms, classes, orders, and families have a real existence, and that it is possible to ascertain and distinguish characters which are of class value, others which are only of ordinal value, and so on, so that the classes of one sub-kingdom should on paper, and in nature actually do, correspond in relative value to those of another sub-kingdom, and the orders of any one class similarly should be so taken as to be of equal value with those of another class, and have been actually so created.

The whole position was changed by the acquiescence, which became universal, in the doctrine of Darwin. That doctrine took some few years to produce its effect, but it became evident at once to those who accepted Darwinism that the natural classification of animals, after which collectors and anatomists, morphologists, philosophers, and embryologists had been so long striving was nothing more nor less than a genealogical tree, with breaks and gaps of various extent in its record. The facts of the relationships of animals to one another, which had been treated as the outcome of an inscrutable law by most zoologists and glibly explained by the transcendental morphologists, were amongst the most powerful arguments in support of Darwin’s theory, since they, together with all other vital phenomena, received a sufficient explanation through it. It is to be noted that, whilst the zoological system took the form of a genealogical tree, with main stem and numerous diverging branches, the actual form of that tree, its limitation to a certain number of branches corresponding to a limited number of divergencies in structure, came to be regarded as the necessary consequence of the operation of the physico-chemical laws of the universe, and it was recognized that the ultimate explanation of that limitation is to be found only in the constitution of matter itself.

The first naturalist to put into practical form the consequences of the new theory, in so far as it affected zoological classification, was Ernst Haeckel of Jena (b. 1834), who in 1866, seven years after the publication of Darwin’s Origin of Species, published his suggestive Generelle Morphologie. Haeckel introduced into classification a number of terms intended to indicate the branchings of a genealogical tree. The whole “system” or scheme of classification was termed a genealogical tree (Stammbaum) ; the main branches were termed “phyla,” their branchings “subphyla”; the great branches of the sub-phyla were termed “cladi,” and the “cladi” divided into “classes,” these into sub-classes, these into legions, legions into orders, orders into sub-orders, sub-orders into tribes, tribes into families, families into genera, genera into species. Additional branchings could be indicated by similar terms where necessary. There was no attempt in Haeckel’s use of these terms to make them exactly or more than approximately equal in significance; such attempts were clearly futile and unimportant where the purpose was the exhibition of lines of descent, and where no natural equality of groups was to be expected ex hypothesi. Haeckel’s classification of 1866 was naturally enough only a first attempt. In the edition of the Natürliche Schöpfungsgeschichte published in 1868, he made a great advance in his genealogical classification, since he now introduced the results of the extraordinary activity in the study of embryology which followed on the publication of the Origin of Species.

The pre-Darwinian systematists since the time of Von Baer had attached very great importance to embryological facts, holding that the stages in an animal’s development were often more significant of its true affinities than its adult structure. Von Baer had gained unanimous support for his dictum, “Die Entwickelungsgeschichte ist der wahre Lichtträger für Untersuchungen über organische Körper.” Thus J. Müller’s studies on the larval forms of Echinoderms and the discoveries of Vaughan Thompson were appreciated. But it was only after Darwin that the cell-theory of Schwann was extended to the embryology of the animal kingdom generally, and that the knowledge of the development of an animal became a knowledge of the way in which the millions of cells of which its body is composed take their origin by fission from a smaller number of cells, and these at last from the single egg-cell. Kölliker {Development of Cephalopods, 1844), Remak {Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kowalewsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier. Kowalewsky’s work appeared between the dates of the Generelle Morphologie and the Schöpfungsgeschichte. Haeckel himself, with his pupil Miklucho-Maclay, had in the meantime made studies on the growth from the egg of Sponges,—studies which resulted in the complete separation of the unicellular or equicellular Protozoa from the Sponges, hitherto confounded with them. It is this introduction of the consideration of cell-structure and celldevelopment which, subsequently to the establishment of Darwinism, has most profoundly modified the views of systematists, and led in conjunction with the genealogical doctrine to the greatest activity in research,—an activity which culminated in the work (1873-82) of F. M. Balfour, and produced the profoundest modifications in classification.

Haeckel’s earlier pedigree is worth comparing with his second effort, as showing the beginning of the influence just noted. The second pedigree is as follows :— [table]

In representing pictorially the groups of the animal kingdom as the branches of a tree, it becomes obvious that a distinction may be drawn, not merely between the individual main branches, but further as to the level at which they are given off from the main stem, so that one branch or set of branches may be marked off as belonging to an earlier or lower level than another set of branches; and the same plan may be adopted with regard to the clades, classes, and smaller branches. The term “grade” was introduced by Lankester to indicate this giving off of branches at a higher or lower, i.e., a later or earlier, level of a main stem. The mechanism for the statement of the genealogical relationships of the groups of the animal kingdom was thus completed. Renewed study of every group was the result of the acceptance of the genealogical idea and of the recognition of the importance of cellular embryology. On the one hand, the true method of arriving at a knowledge of the genealogical tree was recognized as lying chiefly in attacking the problem of the genealogical relationships of the smallest twigs of the tree, and proceeding from them to the larger branches. Special studies of small families or orders of animals with this object in view were taken in hand by many zoologists. On the other hand, a survey of the facts of cellular embryology which were accumulated in regard to a variety of classes within a few years of Kowalewsky’s work led to a generalization, independently arrived at by Haeckel and Lankester, to the effect that a lower grade of animals may be distinguished, the Protozoa or Plastidozoa, which consist either of single cells or colonies of equiformal cells, and a higher grade, the Metazoa or Enterozoa, in which the egg-cell by “cell division” gives rise to two layers of cells, the endoderm and the ectoderm, surrounding a primitive digestive chamber, the archenteron. Of these latter, two grades were further distinguished by Lankester,—those which remain possessed of a single archenteric cavity and of two primary cell-layers (the Coelentera or Diploblastica), and those which by nipping off the archenteron give rise to two cavities, the coelom or body-cavity and the metenteron or gut (Coelomata or Triploblastica). To the primitive two-cell-layered form, the hypothetical ancestor of all Metazoa or Enterozoa, Haeckel gave the name Gastraea; the embryonic form which represents in the individual growth from the egg this ancestral condition he called a “gastrula.” The term “diblastula” has more recently been adopted in England for the gastrula of Haeckel. The tracing of the exact mode of development, cell by cell, of the diblastula, the coelom, and the various tissues of examples of all classes of animals has been pursued during the last twenty years with immense activity and increasing instrumental facilities, and is still in progress.

Two names in connexion with post-Darwinian taxonomy and the ideas connected with it require brief mention here. Fritz Miiller, by his studies on Crustacea (Für Darwin, 1864), showed the way in which genealogical theory may be applied to the minute study of a limited group. He is also responsible for the formulation of an important principle, called by Haeckel “the biogenetic fundamental law,” viz., that an animal in its growth from the egg to the adult condition tends to pass through a series of stages which are recapitulative of the stages through which its ancestry has passed in the historical development of the species from a primitive form; or, more shortly, that the development of the individual (ontogeny) is an epitome of the development of the race (phylogeny). Pre-Darwinian zoologists had been aware of the class of facts thus interpreted by Fritz Müller, but the authoritative view on the subject had been that there is a parallelism between (a) the series of forms which occur in individual development, (δ) the series of existing forms from lower to higher, and (c) the series of forms which succeed one another in the strata of the earth’s crust, whilst an explanation of this parallelism was either not attempted, or was illusively offered in the shape of a doctrine of harmony of plan in creation. It was the application of Fritz Müller’s law of recapitulation which gave the chief stimulus to recent embryological investigations; and, though it is now recognized that “recapitulation” is vastly and bewilderingly modified by special adaptations in every case, yet the principle has served, and still serves, as a guide of great value.

Another important factor in the present condition of zoological knowledge as represented by classification is the doctrine of degeneration propounded by Anton Dohm. Lamarck believed in a single progressive series of forms, whilst Cuvier introduced the conception of branches. The first post-Darwinian systematists naturally and without reflexion accepted the idea that existing simpler forms represent stages in the gradual progress of development,—are in fact survivors from past ages which have retained the exact grade of development which their ancestors had reached in past ages. The assumption made was that (with the rare exception of parasites) all the change of structure through which the successive generations of animals have passed has been one of progressive elaboration. It is Dohrn’s merit to have pointed out that this assumption is not warranted, and that degeneration or progressive simplification of structure may have, and in many lines certainly has, taken place, as well as progressive elaboration and continuous maintenance of the status quo. The introduction of this conception necessarily has had most important effect in the attempt to unravel the genealogical affinities of animals. It renders the task a more complicated one; at the same time it removes some serious difficulties and throws a flood of light on every group of the animal kingdom.

One result of the introduction of the new conceptions dating from Darwin has been a healthy reaction from that attitude of mind which led to the regarding of the classes and orders recognized by authoritative zoologists as sacred institutions which were beyond the criticism of ordinary men. That state of mind was due to the fact that the groupings so recognized did not profess to be simply the result of scientific reasoning, but were necessarily regarded as the expressions of the “insight” of some more or less gifted persons into a plan or system which had been arbitrarily chosen by the Creator. Consequently there was a tinge of theological dogmatism about the whole matter. To deny the Linnaean, or later the Cuvierian, classes was very much like denying the Mosaic cosmogony. At the present time systematic zoology is entirely free from any such prejudices, and the Linnaean taint which is annarent even in Haeckel and Gegenbaur may be considered as finally expunged.

We give below the classification of Ray Lankester as an example of the most recent genealogical classification. It is represented by the above genealogical tree and the tabular statement which follows. The chief points in this classification are the inclusion of Balanoglossus and the Tunicata in the phylum Vertebrata, the association of the Rotifera and the Chaetopoda with the Arthropoda in the phylum Appendiculata, the inclusion of Limulus and the Eurypterina in the class Arachnida, and the total abandoning of the indefinite and indefensible group of “Femes.”

Grade A. PLASTIDOZOA (PROTOZOA). [table] [table]

[table] [table]

We have now traced the history of the morphography of animals so as to show that increasingly in successive epochs independent branches of knowledge have been brought to bear on the consideration of the main problem, namely, the discrimination of the kinds and the relations to one another of animal forms. Before glancing at the history of the remaining branches of zoological science, which have had an independent history whilst ultimately contributory to taxonomy and morphography, it may be briefly pointed out that the accumulation of knowledge with regard to the distribution of animal forms on the earth’s surface and in the seas has progressed simultaneously with the discrimination of the mere forms of the species themselves, as has also the knowledge derived from fossilized remains as to the characters of former inhabitants of the globe. Both these subdivisions of morphography have contributed to the establishment of Darwinism,—the one (palaeontology) by direct evidence of organic evolution in time, the other (zoo-geography) in a more indirect way.

Alfred Russell Wallace stands prominently forward as a naturalist-traveller who by his observations, chiefly on Lepidopterous Insects, in both South America and the Malay Archipelago, was led to the conclusion that a production of new species is actually going on, and that, too, by means of a process of natural selection of favourable variations. Wallace and Darwin, who each recognized cordially and fully the other’s work, laid their views before the Linnean Society on the same day in 1859.

The facts of the geographical distribution of animals were systematized, and great zoo-geographical provinces first clearly recognized, by P. L. Sclater in 1857. The application of the Darwinian theory to the facts tabulated by Sclater, combined with a knowledge of the distribution of animals in past geological periods, has led to a full explanation of the migrations of terrestrial animals, and has furnished a striking corroboration of the sufficiency of the doctrine of organic evolution, as reformed by Darwin, to account for all the phenomena of zoology.

The study of the marine fauna by means of the dredge and trawl had been enthusiastically prosecuted by British, French, and Scandinavian naturalists in the two decades before Darwin’s book; the collection of forms, the discovery of new species, and the recording of their bathymetrical and local distribution had produced a great mass of knowledge through the labours of E. Forbes, Gwyn-Jeffreys, Sars, Quatrefages, Norman, and others. The post-Darwinian developments of this line of inquiry have been two. In the first place, dredging and trawling have been extended by the aid of steamships of the Norwegian, British, American, French, and Italian navies into greater depths than were previously supposed to contain living things. New species and genera, and a vast extension of knowledge as to distribution, have been the outcome of these expeditions, connected with the names of G. O. Sars and Daniellsen in Norway, of Alex. Agassiz in America, and of Carpenter and Wyville Thomson in Great Britain. It is worthy of note that the practical demand for sounding the Atlantic in connexion with the laying of the first deep-sea telegraphcable is what led to these explorations, the first recognition of life at these great depths in the ocean being due to Dr Wallich, who accompanied a sounding expedition in 1860 to the North Atlantic, and to Prof. Fleeming Jenkin, who in the same year acted as engineer in raising the submarine cable between Sardinia and Africa, upon which living corals were found. In the second place, the study of marine zoology has, since the publication of the Origin of Species, been found to require more complete arrangements in the form of laboratories and aquaria than the isolated vacation student could bring with him to the seaside. Seaside laboratories have come into existence: the first was founded in France by Coste (1859) at Concarneau (Brittany), again with a practical end in view, viz., the study of food-fishes with an aim to pisciculture. The demand for a knowledge of the embryology of all classes of animals, and for further facts as to the structure and lifehistory of the minuter microscopic or very delicate forms of marine life, is what has determined the multiplication of these marine “stations.” The largest and best supported pecuniarily is that founded at Naples by Anton Dohm in 1872; others exist at Trieste, Villefranche, Cette, and at New Haven and Beaufort in the United States, whilst a large laboratory, on a scale to compare with that at Naples, has this year (1888) been opened at Plymouth by the Marine Biological Association of the United Kingdom.

Another result of the stimulus given to zoological research by Darwin’s work is the undertaking of voyages to distant lands by skilled anatomists for the purpose of studying on the spot, and with all the advantages of abundant and living material, the structure, and especially the embryology, of rare and exceptionally interesting forms of animal life. In the pre-Darwinian period of this century zoologists who were convinced of the importance of anatomical and embryological study were still content to study specimens immersed in spirit and brought home, often imperfectly preserved, by unskilled collectors, or to confine their attention to such species as could be procured in Europe. Before Cuvier, as we have already pointed out, attention was, with rare exceptions, limited to the dried skeletons and external forms of animals. Now, however, the enterprising zoologist goes to the native land of an interesting animal, there to study it as fully as possible. The most important of these voyages has been that of W. H. Caldwell of Cambridge to Australia (1885-86) for the purpose of studying the embryology of the Monotrema and of Ceratodus, the fish-like Dipnoon, which has resulted in the discovery that the Monotrema are oviparous. Similarly Adam Sedgwick proceeded to the Cape in order to study Peripatus, Bateson to the coast of Maryland to study Balanoglossus, and the brothers Sarassin to Ceylon to investigate the embryology of the Coecilia.

The task of the zoologist has changed and developed in every succeeding period. Pure morphography has long ceased to be a chief line of research; and now even the preoccupation produced by the addition to it of the study of cellular embryology is about to undergo a modification by the demand for knowledge of the facts of heredity and adaptation in greatly extended detail.

Zoo-Mechanics, Zoo-Physics, Zoo-Chemistry.

The development of that knowledge of the structure of the human body, and of the chemical and physical processes going on in it, which is necessary for the purposes of the medical art forms a distinct history, which has both influenced and been influenced by that of other branches of zoology. The study of the structure and composition of the body of man and of the animals nearest to him was until fifty years ago one with the inquiry into the activities of those parts, and indeed the separation of anatomy and physiology has never been really carried out. For convenience of teaching, the description of the coarser anatomy of the human body has been in modern universities placed in the hands of a special professor, theoretically condemned to occupy himself with the mere formal details of structure, whilst the professor of physiology has usually retained what is called “microscopic anatomy,” and necessarily occupies himself with as much structural anatomy as is required for a due description of the functions of organs and the properties of tissues. It would seem that in our medical schools and universities these arrangements should be reconsidered. Anatomy and physiology should be re-united and subdivided as follows,—(1) physiology with anatomy in relation to physiology, (2) anatomy in relation to surgery and medical diagnosis,—the former being a science, the latter a piece of technical training in rule of thumb.

Physiological anatomy or anatomical physiology has its beginnings in Aristotle and other observers of antiquity. The later Graeco-Roman and the Arabian physicians carried on the traditional knowledge and added to it. Galen dominated the Middle Ages. The modern development begins with Harvey and with the Italian school in which he studied. Its great names are Fabricius of Acquapendente (1537-1619), Vesalius (1514-1564), Eustachius (c. 1500- 1574), Riolan (1577-1657), Severino (1580-1656). The history of the discovery of the circulation of the blood and of the controversies connected with it gives an interesting and sufficient presentation of the anatomico-physiological knowledge of the period (see Harvey). The foundation of the scientific academies and the records of their publications furnish thenceforward a picture of the progress in this study. As an early anatomist Willis (1621-1675), professor of physic in Oxford, deserves notice for his work on the anatomy of the human brain, the plates for which were drawn by young Christopher Wren, the prodigy of Oxford common-rooms, who later built St Paul’s Cathedral. The Royal Society, in its early days when Wren was a fellow, met at Gresham College whenever the professor of physic there could obtain a human body for dissection, and amongst its earliest records are the memoirs of Tyson on the anatomy of the Chimpanzee and the experiments on transfusion of blood, extirpation of the spleen, and such like inquiries.

Marcello Malpighi (1628-1694) and Anton van Leeuwenhoek (1632-1723) were the first to introduce the microscope into anatomical research. Malpighi first used the injection of blood-vessels on a large scale, and moreover is to be credited with having first conceived that there is a definite relation of the structure of lower kinds of animals to that of higher and more elaborate kinds, and that this relation is one of gradual transition, so that lower animals are not to be regarded as isolated and arbitrary existences, but are really simpler exhibitions of the same kind of structure and mechanism which occurs in higher animals. It is this conception which later developed into the theory of an actual transmutative development of lower into higher organisms. Leeuwenhoek discovered the red blood corpuscles of Vertebrates, saw the circulation in the capillaries of the Frog’s foot, described the fibrillar structure and cross-striping of muscular fibre, the tubular structure of dentine, the scales of the epidermis, the fibres of the lens, and the spermatozoa, these last having been independently discovered at Leyden in 1677 by Ludwig Ham of Stettin. The spermatozoa were regarded by the “animalculists” as the fully formed but minute young which had to be received in the egg, in order to be nourished and increase in size, and were hailed as a decisive blow to Harvey’s doctrine of epigenesis and his dictum “omne vivum ex ovo.” Albrecht von Haller was the champion of the so-called “evolutionists” in the 18th century, better called “praeformationists.” Haller wrote, “There is no such thing as development! No part of the animal body is made before another; all are simultaneously created.” A corollary of this doctrine was that the germ contains the germs of the next generation, and these of the next, and so ad infinitum. It was calculated that Eve at her creation thus contained within her 200,000 millions of human germs. This was the view of the “ovists,” who regarded the egg as the true germ, whilst the “animalculists,” who regarded the spermatozoon as the essential germ, would have substituted Adam for Eve in the above calculation. These fanciful conceptions —containing as they do a share of important truth—were opposed by Caspar Friedrich Wolff, who in his doctorate dissertation (1759) maintained that the germ is a structureless particle, and acquires its structure by “epigenesis” or gradual development. Wolff has proved to be nearer the truth than Haller; but modern conceptions as to the molecular structure of the egg-protoplasm point to a complexity as great as that imagined by the evolutionists. Later it was maintained that the spermatozoa are parasitic animalcules, and this view prevailed for 150 years, so that in the Physiology of Johann Müller (1842) we read, “Whether the spermatozoa are parasitic animalcules or living parts of the animal in which they occur cannot at present be stated with certainty.”

Physiology in the 18th century could only proceed by means of inferences from purely anatomical observation, aided by imaginative conceptions which had no real basis. The explanation of the processes of life in the animal body was waiting for that progress in the knowledge of physics and chemistry which at last arrived, and gave a new impulse to investigation. Albrecht von Haller (1708-1777) was the first to apply experimental methods to the determination of the functions of the various organs made known by anatomists, and from him we may trace a bifurcation in the tendencies of medical men who occupied themselves with the study of the structure and functions of the animal organism. The one class proceeded more and more in the direction of comparative anatomy, the other in the direction of exact analysis and measurement of both the structure and properties of the organs of Vertebrate animals allied to man and of man himself.

John Hunter (1728-1793) is the most striking figure of this epoch in the relation of medicine to general zoological progress; the preservation of his museum in Lincoln’s Inn Fields, London, by the combined action of the state and the Royal College of Surgeons, is an abiding record of the historical progress of biological science. Hunter collected, dissected, and described not only higher but lower animals, with the view of arriving at a knowledge of the function of organs by the most extensive and systematic survey of their modifications in all kinds of animals. His purpose was that of the physiologist and medical man, but he made great contributions to the general knowledge of animal structure. The same class of investigations, when taken up by Cuvier from the point of view of systematic zoology and morphology, led to a reconstruction of classification and laid the foundation of anatomical zoology. Hunter was the younger brother of William Hunter, who also formed an important museum, still preserved in Glasgow. Hunter classified the organs of animals into those which subserve the preservation of the individual, those which subserve the preservation of the species, and those which are the means of relation with the outer world, and he arranged his museum of dissections and preparations on this plan.

The great progress of chemistry at the end of the 18th and the beginning of the 19th century was followed by an application of chemical laws and chemical methods to the study of animal life. Curiously enough, as showing how deeply interwoven are the various lines of scientific progress, Priestley in his discovery of oxygen was as much concerned in the study of a chlorophyll-bearing Protozoon, Euglena viridis, as in that of the red oxide of mercury; and the interest in “vital spirits” as a physiological factor was an important stimulus to those researches which produced modern chemical knowledge.

The purely anatomical side of physiological progress is marked in the beginning of the 19th century by the work of Bichat (1771-1802), who distinguished by naked-eye characters the different structural materials of which the organs of man and the higher animals are built, and thus founded in first outline the science of histology. By the end of the first quarter of this century it had become clear to the minds of the anatomico-physiological students of animal life that the animal body was subject to the same physical laws as other matter, although it was still held that some additional and mysterious agent—so-called “vitality ”—was at work in living bodies. It had become clear that animal material could be investigated chemically, and that the processes of digestion, assimilation, respiration, and secretion were chemical processes.

To a considerable extent the chemical composition and properties of the tissues, and the chemical nature of the various changes of life and of putrefaction after death, had been investigated, but one step was yet to be taken which brings the study of ultimate structure, chemical activity, form, and the formation of form to a single focus. This was taken by Theodore Schwann (1810-1881), who in 1839 published his epoch-making cell-theory. Schwann was a pupil of Johann Müller, and there can be little doubt that the ideas of the pupil are to be credited in some measure to the master. Schwann took up the thread of microscopic investigation which had been sedulously pursued by a distinct line of students since the days of Hook and Leeuwenhoek, and had resulted in a general doctrine among botanists of the cellular structure of all the parts of plants. Schwann showed not only that plants are uniformly built up by these corpuscular units (of which Robert Browne in 1833 had described the peculiar nucleated structure), but that all animal tissues are also so built up. That, however, was not Schwann’s chief point. The cell-theory for which he is famous is this, that the substance of the individual cell is the seat of those chemical processes which seen en masse we call life, and that the differences in the properties of the different tissues and organs of animals and plants depend on a difference in the chemical and physical activity of the constituent cells, resulting in a difference in the form of the cells and in a concomitant difference of activity. Schwann thus pointed to the microscopic cell-unit as the thing to be studied in order to arrive at a true knowledge of the processes of life and the significance of form. In founding the study of cell-substance (or protoplasm, as it was subsequently called by Max Schultze in 1861, adopting the name used by botanists for vegetable cell-contents) Schwann united two lines of inquiry, viz., that of minute investigation of structure and development and that of zoochemistry and zoo-physics. He spent a large part of the next forty years in an attempt to penetrate further into the structure of cell-substance; he hoped to be able to find in cell-substance ultimate visible molecules, a knowledge of the arrangement and characters of which would explain the varying properties of protoplasm.

It is not a little remarkable that Schwann, who thus brought about the union of physiological and morphological study by his conception of cell-substance, should also have been the initiator of that special kind of experimental investigation of the physical properties of tissues by the exact methods used by physicists which, by the aid of the kymographion, the thermo-electric pile, and the galvanometer, has been so largely pursued during the last thirty years in our physiological laboratories. It is perhaps less surprising that Schwann, who had so vivid a conception of the activity and potentialities of the cell-unit, should have been the discoverer of the immensely important fact that putrefaction and fermentation are not the consequences of death but of life, and that without the presence of living Bacteria putrefaction does not occur, whilst he also is the discoverer of the fact that the yeast which causes alcoholic fermentation is a mass of unicellular living organisms.

From Schwann’s time onward the cell became more and more the point of observation and experiment in the progress of both morphography and physiology. It was soon shown, chiefly through Kölliker and Remak, that all cells originate by fission from pre-existing cells,—a fact unknown to Schwann,—and the doctrine “omnis cellula e cellula” was established. It was also demonstrated that the Mammalian egg discovered by Von Baer was a typical nucleated cell, and that all animals, and plants also (this generalization took thirty years to establish), take their origin from an egg, which is in essence and in fact a single nucleated cell. The doctrine of Harvey, “omne vivum ex ovo,” thus received its most ample justification. The study of “growth from the egg” became necessarily a study of the multiplication by fission of the egg-cell and its fission-products, their arrangement in layers, and the chemical metamorphosis of their substance and exudations. This study, as well as the allied investigation of the cell-structure of the adult tissues, was immensely facilitated by methods of hardening, staining, section-cutting, and clarifying which grew up after Schwann’s time, and have their present highest development in the automatic microtome of Caldwell, which can be worked by a motor, and delivers consecutive sections of animal tissues or embryos 1 / 4000 th of an inch thick, arranged in the form of ribbons, ready for examination with the microscope, at the rate of one hundred or more per minute. Stricker of Vienna was the first to embed embryos in waxy material for the purpose of cutting thin sections of them, about twenty-five years ago, and R. Leuckart of Leipsic was subsequently the first to employ this method in the study of the structure of small Invertebrata.

The knowledge of the anatomical facts of cellular development and cellular structure necessarily gave immensely increased precision to the notion of gradation of structure in the animal series from simple to complex, and rendered Darwin’s doctrine the more readily accepted. It was not, however, until after Darwin’s date (1859) that the existence of unicellular animals was fully admitted, and the general facts of cellular embryology established throughout the animal kingdom.

Similarly cellular physiology, by establishing the conception of a simple optically homogeneous cell-substance as the seat of the activities which we call “life,” rendered it possible to accept the suggestion of a simple “substance of life” which might have been evolved from simpler nonliving matter by natural processes depending on physical and chemical laws. It is noteworthy that Darwin himself appears not to have been influenced directly by any such physiological or chemico-physical doctrine as to “protoplasm” or cell-substance. Nevertheless the way was prepared for the reception of Darwin’s theory by this state of physiological knowledge.

The word “protoplasm” requires a little further notice. Protoplasm was applied by Von Mohl and by Max Schultze to the slimy substance of the cell, including therein both the general thinner material and the nucleus. It is, as Roscoe remarked at Manchester (Brit. Ass. Address, 1887), a structure and not a chemical body. Nevertheless gradually physiologists have come to use the word “protoplasm” for one of the chemical substances of which Schultze’s protoplasm is a structural mixture—namely, that highest point in the chemical elaboration of the molecule which is attained within the protoplasm, and up to which some of the chemical bodies present are tending, whilst others are degradation products resulting from a downward metamorphosis of portions of it. This intangible, unstable, all-pervading element of the protoplasm cannot at present be identified with any visibly separable part of the cell-substance, which consists of a hyaline denser network of excessive tenuity, of a less dense hyaline liquid, and of finest and less fine granules of varying chemical nature. This “critical” substance, sometimes called “true protoplasm,” should assuredly be recognized by a distinct name “plasmo-gen,” whilst protoplasm retains its structural connotation.

The study of the process of fertilization and of the significance in that process of the distinct parts of the sperm-cell and egg-cell—the separate fibrillae and granules of the nuclei of those cells—at the present moment forms one of the engrossing subjects of zoological investigation. Not less important is the descent, as it were, of physiological investigation in relation to every organ into the arena of the cell: digestion, secretion, muscular contraction, nerve action, all are now questions of plasmology, or the study of cell-substance founded by Schwann. General Tendency of Zoology since Darwin.

The serious and broadly-based study of bionomics which was introduced by Darwin, and in his hands gave rise to the doctrine of natural selection, by which the hypothesis of the origin of species by gradual transmutation in the natural process of descent from ancestral forms was established as a scientific doctrine, can hardly be said to have had any history. Buffon (1707-1788) alone among the greater writers of the three past centuries emphasized that view of living things which we call “bionomics.” Buffon deliberately opposed himself to the mere exposition of the structural resemblances and differences of animals, and, disregarding classification, devoted his treatise on natural history to a consideration of the habits of animals and their adaptations to their surroundings, whilst a special volume was devoted by him to the subject of reproduction. In special memoirs on this or that animal, and in a subordinate way in systematic works, material is to be found helping to build up a knowledge of bionomics, but Buffon is the only prominent writer who can be accorded historic rank in this study. The special study of man in these relations—such as is concerned with the statistics of population—must be considered as having contributed very importantly to Darwin’s wider study of bionomics in general. The work of Malthus On Population ( 1798 ) exercised the most important influence on Darwin’s thought, as he himself tells us, and led him to give attention to the facts of animal population, and so to discover the great moving cause of natural selection—the struggle for existence. Darwin may be said to have founded the science of bionomics, and at the same time to have given new stimulus and new direction to morphography, physiology, and plasmology, by uniting them as contributories to one common biological doctrine—the doctrine of organic evolution— itself but a part of the wider doctrine of universal evolution based on the laws of physics and chemistry.

The full influence of Darwin’s work upon the progress and direction of zoological study has not yet been seen. The immediate result has been, as pointed out above, a reconstruction of the classification of animals upon a genealogical basis, and an investigation of the individual development of animals in relation to the steps of their gradual building up by cell-division, with a view to obtaining evidence of their genetic relationships. On the other hand, the studies which occupied Darwin himself so largely subsequently to the publication of the Origin of Species, viz., the explanation of animal (and vegetable) mechanism, colouring, habits, &c., as advantageous to the species or to its ancestors—in fact, the new teleology,—has not yet been so vigorously pursued as it must be hereafter. The most important work in this direction has been done by Fritz Müller (Fur Darwin), by Herman Müller (Fertilization of Plants by Insects), and by August Weismann (memoirs translated by Meldola). Here and there observations are from time to time published, but no large progress has yet been made, probably on account of the fact that animals are exceedingly difficult to keep under observation, and that there is no provision in universities and like institutions for the pursuit of these inquiries or even for their academic representation. More has been done with plants than with animals in this way since Darwin, probably owing to the same cause which has, ever since the revival of learning, given botany a real advantage over zoology, namely, the existence of “physick” gardens, now become “botanical” gardens, and the greater ease of management, experiment, and observation in the case of plants than in that of animals. It is true that zoological gardens have existed for the last fifty years in all large European cities, but these have always been conducted with a view to popular exhibition; and, even where scientific influences have been brought to bear on their management, they have been those of the morphographer and systematist rather than of the bionomist. Moreover, zoological gardens have never been part of the equipment of the university professor of zoology, as it may be hoped in future will be the case. The foundation of marine biological laboratories under the control of scientific zoologists offers a prospect of true bionomic observation and experiment on an increased scale in the near future, and, were such laboratories founded in our universities and provided with the necessary appliances for keeping terrestrial and freshwater animals, as well as marine forms, alive and under observation in conditions resembling as nearly as possible those of nature, a step would have been taken towards carrying on the study of bionomics which cannot long be delayed. It seems to be even more important that the academic curriculum of zoology should not, by mere mechanical adhesion to the old lines of morphography, and experimental research on the chemical and physical properties of tissues and organs, confine the attention and training of young students to what are now, comparatively speaking, the less productive lines of research.

If we turn to the other branch of bionomics, that con cerned with the laws of variation and heredity (thremmatology), we find that since Darwin, and independently of his own work, there has been a more obvious progress than in teleology. In the first place, the continued study of human population has thrown additional light on some of the questions involved, whilst the progress of microscopical research in the hands of Bütschli, Hertwig, Balfour, and August Weismann promises to give us a clear foundation as to the structural facts connected with the origin of the egg-cell and sperm-cell and the process of fertilization. This is not the fitting place in which to give a sketch of the doctrines and hypotheses of thremmatology. They may be gathered from Darwin’s writings, more especially the Origin of Species and Animals and Plants under Domestications. They relate to the causes of variation in animals and plants, the laws of the transmission of parental characters, the share of each parent in the production of the characters of the offspring, atavism, and the relations of young to parents as to number, sex, nourishment, and protection.

An important development of Darwin’s conclusions is actually in progress and deserves special notice here, as it is the most distinct advance in the department of bionomics since Darwin’s own writings, and at the same time touches questions of fundamental interest. The matter strictly relates to the consideration of the “causes of variation,” and is as follows.

The fact of variation is a familiar one. No two animals, even of the same brood, are alike: whilst exhibiting a close similarity to their parents, they yet present differences, sometimes very marked differences, from their parents and from one another. Lamarck had put forward the hypothesis that structural alterations acquired by a parent in the course of its life are transmitted to the offspring, and that, as these structural alterations are acquired by an animal or plant in consequence of the direct action of the environment, the offspring inheriting them would as a consequence not unfrequently start with a greater fitness for those conditions than its parents started with. In its turn, being operated upon by the conditions of life, it would acquire a greater development of the same modification, which it would in turn transmit to its offspring. In the course of several generations, Lamarck argued, a structural alteration amounting to such difference as we call “specific” might be thus acquired. The familiar illustration of Lamarck’s hypothesis is that of the giraffe, whose long neck might, he suggested, have been acquired by the efforts of a primitively short-necked race of herbivores, who stretched their necks to reach the foliage of trees in a land where grass was deficient, the effort producing a distinct elongation in the neck of each generation, which was then transmitted to the next. This process is known as “direct adaptation;” and there is no doubt that such structural adaptations are acquired by an animal in the course of its life. Whether such acquired characters can be transmitted to the next generation is a separate question. It was not proved by Lamarck that they can be, and, indeed, never has been proved by actual observation. Nevertheless it has been assumed, and also indirectly argued, that such acquired characters must be transmitted. Darwin’s great merit was that he excluded from his theory of development any necessary assumption of the transmission of acquired characters. He pointed to the admitted fact of congenital variation, and he showed that these variations to all intents and purposes have nothing to do with any characters acquired by the parents, but are arbitrary and, so to speak, non-significant. Their causes are extremely difficult to trace in detail, but it appears that they are largely due to a “shaking up” of the living matter which constitutes the fertilized germ or embryo-cell, by the process of mixture in it of the substance of two cells,—the germ-cell and the sperm-cell,—derived from two different individuals. Other mechanical disturbances may assist in this production of congenital variation. Whatever its causes, Darwin showed that it is all-important. In some cases a pair of animals produce ten million offspring, and in such a number a large range of congenital variation is possible. Since on the average only two of the young survive in the struggle for existence to take the place of their two parents, there is a selection out of the ten million young, none of which are exactly alike, and the selection is determined in nature by the survival of the congenital variety which is fittest to the conditions of life. Hence there is no necessity for an assumption of the perpetuation of direct adaptations. The selection of the fortuitously (fortuitously, that is to say, so far as the conditions of survival are concerned) produced varieties is sufficient, since it is ascertained that they will tend to transmit those characters with which they themselves were born, although it is not ascertained that they could transmit characters acquired on the way through life. A simple illustration of the difference is this; a man born with four fingers only on his right hand is ascertained to be likely to transmit this peculiarity to some at least of his offspring; on the other hand, there is not the slightest ground for supposing that a man who has had one finger chopped off, or has even lost his arm at any period of his life, will produce offspring who are defective in the slightest degree in regard to fingers, hand, or arm. Darwin himself, apparently influenced not merely by the consideration of certain classes of facts which seem to favour the Lamarckian hypothesis but also by a respect for the general prejudice in its favour and for Mr Herbert Spencer’s authority, was of the opinion that acquired characters are in some cases transmitted. It should be observed, however, that Darwin did not attribute an essential part to this Lamarckian hypothesis of the transmission of acquired characters, but expressly assigned to it an entirely subordinate importance.

The new attitude which has been taken since Darwin on this question is to ask for evidence of this asserted transmission of acquired characters. It is held that the Darwinian doctrine of selection of fortuitous congenital variations is sufficient to account for all cases, that the Lamarckian hypothesis of transmission of acquired characters is not supported by experimental evidence, and that the latter should therefore be dismissed. Weismann has also ingeniously argued from the structure of the egg-cell and sperm-cell, and from the way in which, and the period at which, they are derived in the course of the growth of the embryo from the egg—from the fertilized egg-cell— that it is impossible (it would be better to say highly improbable) that an alteration in parental structure could produce any exactly representative change in the substance of the germ or sperm-cells.

It does not seem improbable that the doctrine of organic evolution will thus become pure Darwinism and be entirely dissociated from the Lamarckian heresy.

The one fact which the Lamarckians can produce in their favour is the account of experiments by Brown-Séquard, in which he produced epilepsy in guinea-pigs by section of the large nerves or spinal cord, and in the course of which he was led to believe that in a few rare instances the artificially produced epilepsy was transmitted. This instance does not stand the test of criticism. It is not clear whether the guinea-pigs operated upon had or had not already a constitutional tendency to epilepsy, and it is not clear in what proportion of cases the supposed transmission took place, and whether any other disease accompanied it. On the other hand, the vast number of experiments in the cropping of the tails and ears of domestic animals, as well as of similar operations on man, are attended with negative results. No case of the transmission of the results of an injury can be produced. Stories of tailless kittens, puppies, and calves, born from parents one of whom had been thus injured, are abundant, but they have hitherto entirely failed to stand before examination.

Experimental researches on this question are most urgently needed, but they are not provided for either in the morphographical or physiological laboratories of our universities.

Whilst simple evidence of the fact of the transmission of an acquired character is wanting, the a priori arguments in its favour break down one after another when discussed. The very cases which are advanced as only to be explained on the Lamarckian assumption are found on examination and experiment to be better explained, or only to be explained, by the Darwinian principle. Thus the occurrence of blind animals in caves and in the deep sea was a fact which Darwin himself regarded as best explained by the atrophy of the organ of vision in successive generations through the absence of light and consequent disuse, and the transmission (as Lamarck would have supposed) of a more and more weakened and structurally impaired eye to the offspring in successive generations, until the eye finally disappeared. But this instance is really fully explained by the theory of natural selection acting on congenital fortuitous variations. Many animals are thus born with distorted or defective eyes whose parents have not had their eyes submitted to any peculiar conditions. Supposing a number of some species of Arthropod or Fish to be swept into a cavern or to be carried from less to greater depths in the sea,—those individuals with perfect eyes would follow the glimmer of light and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected. In every succeeding generation this would be the case, and even those with weak but still seeing eyes would in the course of time escape, until only a pure race of eyeless or blind animals would be left in the cavern or deep sea. Experiments and inquiries with regard to this subject are in progress; amongst those who have occupied themselves with it are August Weismann of Freiburg and E. B. Poulton of Oxford.

It has been argued that the elaborate structural adaptations of the nervous system which are the corporeal correlatives of complicated instincts must have been slowly built up by the transmission to offspring of acquired experience, that is to say, of acquired brain structure. At first sight it appears difficult to understand how the complicated series of actions which are definitely exhibited as so-called “instincts” by a variety of animals can have been due to the selection of congenital variations, or can be otherwise explained than by the transmission of habits acquired by the parent as the result of experience, and continuously elaborated and added to in successive generations. It is, however, to be noted, in the first place, that the imitation of the parent by the young possibly accounts for some part of these complicated actions, and, secondly, that there are cases in which curiously elaborate actions are performed by animals as a characteristic of the species, and as subserving the general advantage of the race or species, which, nevertheless, can not be explained as resulting from the transmission of acquired experience, and must be supposed to be due to the natural selection of a fortuitously developed habit which, like fortuitous colour or form variation, happens to prove beneficial. Mr Poulton has insisted upon the habits of “shamming dead” and the combined posturing and colour peculiarities of certain caterpillars (Lepidopterous larvae) which cause them to resemble dead twigs or similar surrounding objects. The advantage to the animal of this imitation of surrounding objects is that it escapes the pursuit of (say) a bird which would, were it not deceived by the resemblance, attack and eat the caterpillar. Now it is clear that preceding generations of caterpillars cannot have acquired this habit of posturing by experience. Either the caterpillar postures and escapes, or it does not posture and is eaten; it is not half eaten and allowed to profit by experience. We seem to be justified in assuming that there are many movements of stretching and posturing possible to caterpillars, and that some caterpillars had a congenital fortuitous tendency to one position, some to another, and, finally, that among all the variety of habitual movements thus exhibited one has been selected and perpetuated because it coincided with the necessary conditions of safety, since it happened to give the caterpillar an increased resemblance to a twig.

The view that instinct is the hereditarily fixed result of habit derived from experience has hitherto dominated all inquiry into the subject, but we may now expect to see a renewed and careful study of animal instincts carried out with the view of testing the applicability to each instance of the pure Darwinian theory without the aid of Lamarckism.

The whole of this inquiry has special importance in regard to mankind, since the great questions of influence of race and family as opposed to the influence of education are at issue. If pure Darwinism is to be accepted, then education has no value in directly affecting the mental or physical features of the race, but only in affecting those of the individual. Were acquired characters really and fully transmitted, then every child born would inherit the knowledge of both its parents more or less completely, and from birth onwards would be able to add to its inherited stock, so that the progress of the race in mental acquirements would be prodigiously more rapid than it is. On the other hand, peculiarities of mind and body established in a race or a family acquire increased significance, for they cannot be got rid of by training, but are bound to reappear if the stock which exhibits them is allowed to breed. It seems that the laws of thremmatology may eventually give to mankind the most precise directions, not only as to how to improve the breeds of plants and animals, but as to how to improve the human stock. It is not a little remarkable that the latest development of zoological science should favour that respect to breeding which is becoming less popular than it was, and should tend to modify the current estimate of the results of education.

The relation of Darwinism to general philosophy and of the history of zoology to philosophical doctrines is one of the most interesting chapters which might be written on the subject of this article. It belongs, however, rather to the history of philosophy than to that of zoology. Undoubtedly the conceptions of mankind at different periods of history with regard to cosmogony, and the relations of God, nature, and man, have had a very marked influence upon the study of zoology, just as in its turn the study of zoology has reacted upon those conceptions.

In this, as in other phases of mental development, the ancient Greeks stand out in the most striking manner as possessing what is sometimes called the modern spirit. The doctrine of evolution is formulated in unmistakable terms by Heraclitus and other philosophers of antiquity. Not only so, but the direct examination of nature, including the various forms of animal life, was practised by Aristotle and his disciples in a spirit which, though not altogether free from prejudice, was yet far more like that which actuated the founders of the Royal Society less than three hundred years ago than anything which was manifested in the two thousand years intervening between that date and the time of Alexander the Great. The study of zoology in the Middle Ages was simply a fantastic commentary on Aristotle and the records of animals in the various books of the Bible, elaborated as part of a peculiar system of mystic philosophy, which has more analogy with the fetichism and totem worship of savage races than with any Greek or modern conceptions. So far as philosophy affected the study of zoology in the beginning of the modern period, its influence was felt in the general acceptance of what has been called the Miltonic cosmogony,— namely, that interpretation of the Mosaic writings which is set forth by the poet Milton, and of which the characteristic is the conception of the creation of existing things, including living things, nearly or just as they are, by a rapid succession of “fiats” delivered by an anthropomorphic Creator. It was not until the end of the 18th century that Schelling (as quoted above) conceived that unity of nature and general law of development which is now called the doctrine of evolution.

In England Erasmus Darwin (Zoonomia, published in 1794-96), in France Lamarck (Philosophie Zoologique, 1809) and Geoffroy Saint-Hilaire (Principes de Philosophie Zoologique, 1830), and in Germany Oken (Lehrbuch der Natur-Philosophie, 1809-11), Goethe (Zur Natur Wiss - ensch., Stuttgart, 1817), and Treviranus (Biologie, 1802-5) were the authors of more or less complete systems of a philosophy of nature in which living things were regarded as the outcome of natural law, that is, of the same general processes which had produced the inanimate universe. The “Natur-philosophen,” as they were called in Germany, demand the fullest recognition and esteem. But, just in proportion as the “Natur-philosophen” failed to produce an immediate effect on the study of zoology by their theory of natural development, so was the doctrine of evolution itself deprived of completeness and of the most important demonstration of its laws by the long-continued delay in the final introduction of biology into the area of that doctrine.

Darwin by his discovery of the mechanical principle of organic evolution, namely, the survival of the fittest in the struggle for existence, completed the doctrine of evolution, and gave it that unity and authority which was necessary in order that it should reform the whole range of philosophy. The detailed consequences of that new departure in philosophy have yet to be worked out. Its most important initial conception is the derivation of man by natural processes from ape-like ancestors, and the consequent derivation of his mental and moral qualities by the operation of the struggle for existence and natural selection from the mental and moral qualities of animals. Not the least important of the studies thus initiated is that of the evolution of philosophy itself. Zoology thus finally arrives through Darwin at its crowning development: it touches and may even be said to comprise the history of man, sociology, and psychology.

Bibliography.— Engelmann, Bibliotheca Historico-Naturalis, vol. i., 1846 (being a list of the separate works and academical memoirs relating to zoology published between 1700 and 1846); Carus and Engelmann, Bibl. Zoologica, Leipsic, 1861 (a similar list of works published between 1846 and 1861); J. V. Caius, Gesch. d. Zoologie, Munich, 1872; and L. Agassiz, An Essay on Classification, London, 1859.