“Et primò, quidem, quoniam per epigenesin sive partium suρer-exorientium additamentum pullum fabricari certum est: quaenam pars ante alias omnes exstruatur, et quid de illa ejusque generandi modo observandum veniat, dispiciemus. Ratum sane est et in ovo manifesté apparet quod A ristoteles de perfectorum animalium generatione enuntiat: nimirum, non omnes partes simul fieri, sed ordine aliam post aliam; primùmque existere particulam genitalem, cujus vntute postea (tanquam ex principio quodam) reliquae omnes partes prosiliant. Qualem in plantarum seminibu3 (fabis, putà, aut glandibus) gemmam sive apicem protuberantem cernimus, totius futurae arboris principium. Estque hoec particula velut filius emancipatus seorsurnque collocatus, et principium per se vivens; unde postea membrorum ordo describitur; et quaecunque ad absolvendum animal pertinent, disponuntur . Quoniam enim nulla pars se ipsam generat; sed postquam generata est, se ipsam jam auget; ideo eam primum oriri necesse est, quae principium augendi contineat (sive enim planta, sive animal est, oeque omnibus inest quod vim habeat vegetandi, sive nutriendi), simulque reliquas omnes partes suo quamque ordine distinguat et formet; proindeque in eadem primogenita particula anima primario inest, sensus, motusque, et totius vitae auctor et principium.” (Exercitatio 51.)

Harvey proceeds to contrast this view with that of the “Medici,” or followers of Hippocrates and Galen, who, “badly philosophizing,” imagined that the brain, the heart, and the liver were simultaneously first generated in the form of vesicles; and, at the same time, while expressing his agreement with Aristotle in the principle of epigenesis, he maintains that it is the blood which is the primal generative part, and not, as Aristotle thought, the heart.

In the latter part of the 17th century, the doctrine of epigenesis thus advocated by Harvey was controverted on the ground of direct observation by Malpighi, who affirmed that the body of the chick is to be seen in the egg before the punctum sanguineum makes its appearance. But from this perfectly correct observation a conclusion which is by no means warranted was drawn; namely, that the chick as a whole really exists in the egg antecedently to incubation; and that what happens in the course of the latter process is no addition of new parts, “alias post alias natas,” as Harvey puts it, but a simple expansion or unfolding of the organs which already exist, though they are too small and inconspicuous to be discovered. The weight of Malpighi’s observations therefore fell into the scale of that doctrine which Harvey terms metamorphosis, in contradistinction to epigenesis.

The views of Malpighi were warmly welcomed on philosophical grounds by Leibnitz, who found in them a support to his hypothesis of monads, and by Malebranche; while, in the middle of the 18th century, not only speculative considerations, but a great number of new and interesting observations on the phenomena of generation, led the ingenious Bonnet, and Haller, the first physiologist of the age, to adopt, advocate, and extend them.

Bonnet affirms that, before fecundation, the hen’s egg contains an excessively minute but complete chick; and that fecundation and incubation simply cause this germ to absorb nutritious matters, which are deposited in the interstices of the elementary structures of which the miniature chick, or germ, is made up. The consequence of this intussusceptive growth is the “development” or “evolution” of the germ into the visible bird. Thus an organized individual (tout organise) “is a composite body consisting of the original, or elementary , parts and of the matters which have been associated with them by the aid of nutrition;” so that, if these matters could be extracted from the individual (tout), it would, so to speak, become concentrated in a point, and would thus be restored to its primitive condition of a germ; “just as, by extracting from a bone the calcareous substance which is the source of its hardness, it is reduced to its primitive state of gristle or membrane.”

“Evolution” and “development” are, for Bonnet, synonymous terms; and since by “evolution” he means simply the expansion of that which was invisible into visibility, he was naturally led to the conclusion, at which Leibnitz had arrived by a different line of reasoning, that no such thing as generation, in the proper sense of the word, exists in nature. The growth of an organic being is simply a process of enlargement, as a particle of dry gelatine may be swelled up by the intussusception of water; its death is a shrinkage, such as the swelled jelly might undergo on desiccation. Nothing really new is produced in the living world, but the germs which develop have existed since the beginning of things; and nothing really dies, but, when what we call death takes place, the living thing shrinks back into its germ state.

The two parts of Bonnet’s hypothesis, namely, the doctrine that all living things proceed from pre-existing germs, and that these contain, one inclosed within the other, the germs of all future living things, which is the hypothesis of “emboîtement;” and the doctrine that every germ contains in miniature all the organs of the adult, which is the hypothesis of evolution or development, in the primary senses of these words, must be carefully distinguished. In fact, while holding firmly by the former, Bonnet more or less modified the latter in his later writings, and, at length, he admits that a “germ” need not be an actual miniature of the organism; but that it may be merely an “original preformation” capable of producing the latter.

But, thus defined, the germ is neither more nor less than the “particula genitalis” of Aristotle, or the “primordium vegetale” or “ovum” of Harvey; and the “evolution” of such a germ would not be distinguishable from “epigenesis.”

Supported by the great authority of Haller, the doctrine of evolution, or development, prevailed throughout the whole of the 18th century, and. Cuvier appears to have substantially adopted Bonnet’s later views, though probably he would not have gone all lengths in the direction of “emboîtement.” In a well-known note to Laurillard’s Éloge, prefixed to the last edition of the Osserneηs fossiles, the “radical de l’être” is much the same thing as Aristotle's “particula genitalis” and Harvey’s “ovum.”

Bonnet’s eminent contemporary, Buffon, held nearly the same views with respect to the nature of the germ, and expresses them even more confidently.

“Ceux qui ont era que le coeur étoit le premier formé, se sont trompés; ceux qui disent que c’est le sang se trompent aussi: tout est formé en même temps. Si l’on ne consulte que l’observation, le poulet se voit dans l’oeuf avant qui’il ait été couvé.”

“J’ai ouvert une grande quantité d’oeufs à differens temps avant et après l’incubation, et je me suis convaincu par mes yeux que le poulet existe en entier dans le milieu de la cicatrale au moment qu’il sort du corps de la poule.”

The “moule intérieur” of Buffon is the aggregate of elementary parts which constitute the individual, and is thus the equivalent of Bonnet?, germ, as defined in the passage cited above. But Buffon further imagined that innumerable “molecules organiques” are dispersed throughout the world, and that alimentation consists in the appropriation by the parts of an organism of those molecules which are analogous to them. Growth, therefore, was, on this hypothesis, partly a process of simple evolution, and partly of what has been termed syngenesis. Buffon’s opinion is, in fact, a sort of combination of views, essentially similar to those of Bonnet, with others, somewhat similar to those ot ’ the “Medici” whom Harvey condemns. The “molecules organiques” are physical equivalents of Leibnitz’s “monads.”

It is a striking example of the difficulty of getting people to use their own powers of investigation accurately, that this form of the doctrine of evolution should have held its ground so long; for it was thoroughly and completely exploded, not long after its enunciation, by Caspar Frederick Wolff, who in his Theoria Generationis, published in 1759, placed the opposite theory of epigenesis upon the secure foundation of fact, from which it has never been displaced. But Wolff had no immediate successors. The school of Cuvier was lamentably deficient in embryologists; and it was only in the course of the first thirty years of the present century, that Prévost and Dumas in France, and, later on, Döllinger, Pander, Von Bär, Rathke, and Remak in Germany, founded modern embryology; and, at the same time, proved the utter incompatibility of the hypothesis of evolution as formulated by Bonnet and Haller, with easily demonstrable facts.

Nevertheless, though the conceptions originally denoted by “evolution” and “development” were shown to bo untenable, the words retained their application to the process by which the embryos of living beings gradually make their appearance; and the terms “Development,” “Entwickelung,” and “Evolutio” are now indiscriminately used for the series of genetic changes exhibited by living beings, by writers who would emphatically deny that “Development” or “Entwickelung” or “Evolutio,” in the sense in which these words were usually employed by Bonnet or Haller, ever occurs.

Evolution, or development, is, in fact, at present employed in biology as a general name for the history of the steps by which any living being has acquired the morphological and the physiological characters which distinguish it. As civil history may be divided into biography, which is the history of individuals, and universal history, which is the history of the human race, so evolution falls naturally into two categories,—the evolution of the individual, and the evolution of the sum of living beings. It will be convenient to deal with the modern doctrine of evolution under these two heads.

1. The Evolution of the Individual.

No exception is, at this time, known to the general law, established upon an immense multitude of direct observations, that every living thing is evolved from a particle of matter in which no trace of the distinctive characters of the adult form of that living thing is discernible. This particle is termed a germ. Harvey says—

“Omnibus viventibus primordium insit, ex quo et a quo proveniant. Liceat hoc nobis primordium végétale nominare; nempe substantiam quandam corpoream vitam habentem potentia; vel quoddam per se existens, quod aptum sit, in vegetativam formam, ab interno principio operante, mutari. Quale nempe primordium, ovum est et plantarum semen; tale etiam viviparorum conceptus et insectorum vermis ab Aristotele dictus: diversa scilicet diversorum viventium primordia.”

The definition of a germ as “matter potentially alive, and having within itself the tendency to assum ∣ definite living form,” appears to meet all the reqιr.ements of modern science. For, notwithstanding it might be justly questioned whether a germ is not merely potentially, but rather actually, alive, though its vital manifestations are reduced to a minimum, the term “potential” may fairly be used in a sense broad enough to escape the objection. And the qualification of “potential” has the advantage of reminding us that the great characteristic of the germ is not so much what it is, but what it may, under suitable conditions, become. Harvey shared the belief of Aristotle—whose writings he so often quotes, and of whom he speaks as his precursor and model, with the generous respect with which one genuine worker should regard another—that such germs may arise by a process of “equivocal generation” out of not-living matter; and the aphorism so commonly ascribed to him, " omne vivum ex ovo,” and which is indeed a fair summary of his reiterated assertions, though incessantly employed against the modern advocates of spontaneous generation, can be honestly ≡o used only by those who have never read a score of pages of the Exercitationes. Harvey, in fact, believed as implicitly as Aristotle did in the equivocal generation of the lower animals. But, while the course of modern investigation has only brought out into greater prominence the accuracy of Harvey’s conception of the nature and mode of development of germs, it has as distinctly tended to disprove the occurrence of equivocal generation, or abiogenesis, in the present course of nature. In the immense majority of both plants and animals, it is certain that the germ is not merely a body in which life is dormant or potential, but that it is itself simply a detached portion of the substance of a pre-existing living body; and the evidence has yet to be adduced which will satisfy any cautious reasoner that

“omne vivum ex vivo” is not as well established a law of the existing course of nature as “omne vivum ex ovo.”

-Hn all instances which have yet been investigated, the substance of this germ has a peculiar chemical composition, consisting of at fewest four elementary bodies, viz., carbon, hydrogen, oxygen, and nitrogen, united into the ill-defined compound known as protein, and associated with much water, and very generally, if not always, with sulphur and phosphorus in minute proportions. Moreover, up to the present time, protein is known only as a product and constituent of living matter. Again, a true germ is either devoid of any structure discernible by optical means, or, at most, it is a simple nucleated cell.

In all cases, the process of evolution consists in a succession of changes of the form, structure, and functions of the germ, by which it passes, step by step, from an extreme simplicity, or relative homogeneity, of visible structure, to a greater dr less degree of complexity or heterogeneity; and the course of progressive differentiation is usually accompanied by growth, which is effected by intussusception. This intussusception, however, is a very different process from that imagined either by Buffon, or by Bonnet. The substance by the addition of which the germ is enlarged is, in no case, simply absorbed ready-made from the not-living world and packed between the elementary constituents of the germ, as Bonnet imagined; still less does it consist of the “molecules organiques” of Buffon. The new material is, in great measure, not only absorbed but assimilated, so that it becomes part and parcel of the molecular structure of the living body into which it enters. And, so far from the fully developed organism being simply the germ plus the nutriment which it has absorbed, it is probable that the adult contains neither in form, nor in substance, more than an inappreciable fraction of the constitutents of the germ, and that it is almost wholly made up of assimilated and metamorphosed nutriment. In the great majority of cases, at any rate, the full grown organism becomes what it is by the absorption of not-living matter, and its conversion into living matter of a specific type. As Harvey says (Ex. 45), all parts of the body are nourished “ab eodem succo alibili, aliter aliterque cambiato,” “ut plantae omnes ex eodem communi nutrimento (sive rore seu terrae humore).”

In all animals and plants above the lowest, the germ is a nucleated cell, using that term in its broadest sense; and the first step in the process of the evolution of the individual is the division of this cell into two or more portions. The process of division is repeated, until the organism, from being unicellular, becomes multicellular. The single cell becomes a cell-aggregate; and it is to the growth and metamorphosis of the cells of the cell-aggregate thus produced, that all the organs and tissues of the adult owe their origin.

In certain animals belonging to every one of the chief groups into which the Metazoa are divisible, the cells of the cell-aggregate which results from the process of yelk division, and which is termed a morula, diverge from one another in such a manner as to give rise to a central space, around which they dispose themselves as a coat or envelope; and thus the morula becomes a vesicle filled with fluid, the planula. The wall of the planula is next pushed in on one side, or invaginated, whereby it is converted into a double walled sac with an opening, the blastospore, which leads into the cavity lined by the inner wall. This cavity is the primitive alimentary cavity, or archenterom · the inner, or invaginated, layer is the hypoblast, the outer the epiblast; and the embryo, in this stage, is termed a gastrula. In all the higher animals, a layer of cells makes its appearance between the hypoblast and the epiblast, and is termed the mesoblast. In the further course of development, the epiblast becomes the ectoderm or epidermic layer of the body; the hypoblast becomes the epithelium of the middle portion of the alimentary canal; and the mesoblast gives rise to all the other tissues, except the central nervous system, which originates from an ingrowth of the epiblast.

With more or less modification in detail, the embryo has been observed to pass through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief, that all animals of higher organization than the Protozoa agree in the general character of the early stages of their individual evolution. Each, starting from the condition of a simple nucleated cell, becomes a cell-aggregate; and this passes through a condition which represents the gastrula stage, before taking in the features distinctive of the group to which it belongs. Stated in this form, the “gastraea theory” of Haeckel appears to the present writer to be one of the most important and best founded of recent generalizations.

So far as individual plants and animals are concerned, therefore, evolution is not a speculation but a fact; and it takes place by epigenesis.

“Animal. . . per epigenesin procreatur, materiam simul attrahit, parat, concoquit, et eadem utitur; formatur simul et augetur. . . . primum futuri corporis concrementum . . . prout augetur, dividitur sensim et distinguitur in partes, non simul omnes, sed alias post alias natas, et ordine quasque suo emergentes.”

In these words, by the divination of genius, Harvey, in the 17th century, summed up the outcome of the work of all those who, with appliances he could not dream of, are continuing his labours in the 19th century.

Nevertheless, though the doctrine of epigenesis, as understood by Harvey, has definitively triumphed over the doctrine of evolution as understood by his opponents of the 18th century, it is not impossible that, when the analysis of the process of development is carried still further, and the origin of the molecular components of the physically gross, though sensibly minute, bodies which we term germs is traced, the theory of development will approach more nearly to metamorphosis than to epigenesis. Harvey thought that impregnation influenced the female organism as a contagion; and that the blood, which he conceived to be the first rudiment of the germ, arose in the clear fluid of the “colliquamentum” of the ovum by a process of concrescence, as a sort of living precipitate. We now know, on the contrary, that the female germ or ovum, in all the higher animals and plants, is a body which possesses the structure of a nucleated cell; that impregnation consists in the fusion of the substance of another more or less modified nucleated cell, the male germ, with the ovum; and that the structural components of the body of the embryo are all derived, by a process of division, from the coalesced male and female germs. Hence it is conceivable, and indeed probable, that every part of the adult contains molecules derived both from the male and from the female parent; and that, regarded as a mass of molecules, the entire organism may be compared to a web of which the warp is derived from the female and the woof from the male. And each of these may constitute one individuality, in the same sense as the whole organism is one individual, although the matter of the organism has been constantly changing. The primitive male and female molecules may play the part of Buffon’s “moules organiques,” and mould the assimilated nutriment, each according to its own type, into innumerable new molecules. From this point of view, the process,...