§ 1. Historical Outline.— If we disregard such vague likenesses as those expressed in the popular classifications of plants by size into herbs, shrubs, and trees, or of terrestrial animals by habit into beasts and creeping things, the history of morphology commences with Aristotle. Founder of comparative anatomy and taxonomy, he established eight great divisions (to which are appended certain minor groups)— Viviparous Quadrupeds, Birds, Oviparous Quadrupeds and Apoda, Fishes, Malakia, Malacostraca, Entoma, and Ostracodermata— distinguishing the first four groups as Enaima (“with blood”) from the remaining four as Anaima (“bloodless”). In these two divisions we recognize the Vertebrata and Invertebrata of Lamarck, while the eight groups are identical with the Mammals, Birds, Reptiles, Fishes, the Cephalopods, Crustaceans, other Articulates, and Testaceans of recent zoology. Far, too, from committing the mistake often attributed to him of reckoning Bats as Birds, or Cetaceans as Fishes, he discerned the true affinities of both, and erected the latter into a special γ∈vos beside the Viviparous Quadrupeds, far more on account of their absence of limbs than of their aquatic habit. Not only is his method inductive, and, as in modern systems, his groups natural, i.e., founded on the aggregate of known characters, but he foreshadows such generalizations as those of the correlation of organs, and of the progress of development from a general to a special form, long afterwards established by Cuvier and Von Baer respectively. In the correspondence he suggests between the scales of Fishes and the feathers of Birds, or in that hinted at between the fins of Fishes and the limbs of Quadrupeds, the idea of homology too is nascent; and from the compilation of his disciple Nicolaus of Damascus, who regards leaves as imperfectly-developed fruits, he seems almost to have anticipated the idea of the metamorphosis of plants. In short, we find a knowledge of structural facts and a comparative freedom from the errors induced by physiological resemblance, of which his successors such as Theophrastus and Pliny, generally mere classifiers by habit, show little trace, and which the moderns have but slowly regained. Little indeed can be recorded until the 13th century, when the reappearance of Aristotle’s works gave a new impulse to the study of organic nature. Of the works of this period that of Albertus Magnus is far the most important; but they are all no more than revivals of Aristotle, marking the reappearance of scientific method and the reawakening of interest in and sympathy with nature. Meanwhile leech and apothecary, alchemist and witch, were accumulating considerable knowledge of plants, which, after the invention of printing, became collected and extended in the descriptive and well-illustrated folios of Gesner and his successors, Fuchs, Lobel, and others, as well as by the establishment of botanic gardens and scientific academies, while, as Sachs expresses it, “in the sharpest contrast to the naive empiricism of the German fathers of botany came their Italian contemporary Caesalpinus, as the thinker of the vegetable world.” Both made systematic efforts,—the Germans vaguely seeking for natural affinities in mere similarities of habit, the Italian with no inconsiderable success striving towards an intellectual basis of classification. Monographs on groups of plants and animals frequently appeared, those of Belon on Birds and Rondelet on Fishes being among the earliest; and in the former of these (1555) we find a comparison of the skeletons of Bird and Man in the same posture and as nearly as possible bone for bone,—an idea which, despite the contemporaneous renaissance of human anatomy initiated by Vesalius, disappeared for centuries, unappreciated save by the surgeon Ambroise Paré. Palissy, like Leonardo before him, discerned the true nature of fossils; and such flashes of morphological insight continued to appear from time to time during the 17th century. Thus, Joachim Jung recognized “the distinction between root and stem, the difference between leaves and foliaceous branches, the transition from the ordinary leaves to the folia floris,” and Harvey anticipated the generalizations of modern embryology by bis researches on development and his theory of epigenesis.

The encyclopaedic period of which Gesner is the highest representative was continued by Aldrovandi, Jonston, and others in the 17th century, but, aided powerfully by the Baconian movement, then profoundly influencing all scientific minds, it developed rapidly into one of genuinely systematic aim. At this stage of progress by far the most important part was taken by John Ray, whose classificatory labours both among plants and animals were crowned with marvellous success. He first definitely expelled the fabulous monsters and prodigies of which the encyclopaedists had faithfully handed on the tradition from mediaeval times, and, like his predecessor Morison, classifying in a truly modern spirit by anatomical characters, he succeeded, particularly among plants, in distinguishing many natural groups, for which his very terms sometimes survive, e.g., Dicotyledons and Monocotyledons, Umbelliferae and Leguminosae. The true precursor of Linnaeus, he introduced the idea of species in natural history, afterwards to become so rigid, and reformed the practice of definition and terminology. Of the many works which followed up Ray’s systematic and monographic labours, though often, like those of Tournefort and Rivinus, Réaumur and Klein, of great importance, none can be even named until we come to those of his great successor Linnaeus, whose extraordinary grasp of logical method and unparalleled lucidity of thought and expression enabled him to reform and reorganize the whole labours of his predecessors into a compact and definite “systema naturae.” The very genius of order, he established modern taxonomy (see Biology), not only by the introduction of the binomial nomenclature and the renovation of descriptive terminology and method, but by the subordination of the species—henceforth clearly defined—under the successive higher categories of genus, order, and class, so finally reconciling the analytic and synthetic tendencies of his predecessors. Although the classification of plants by the number of their essential organs (which vastly advanced not only the cultivation of botany but the knowledge of the flora of the globe, and by which he is popularly remembered) is highly artificial, it must be remembered that this artificiality is after all only a question of degree, and that he not only distinctly recognized its provisional character but collected and extended those fragments of the natural system with which Jussieu soon afterwards commenced to build. His classification of animals, too, was largely natural, and, though on the whole he unfortunately lent his authority to maintain “that disastrous philosophic and scientific aberration” inherited from the alchemists through the last encyclopaedist of Gesner’s school—the notion of three kingdoms of nature —he at least at one time discerned the fundamental unity of animals and vegetables, and united them in opposition to the non-living world as Organisata. At the same time he was still far more a scholastic naturalist than a modern investigator, and his works represent little more than the full completion of the ancient era, and in the hands of fanatical followers served often to retard the commencement of the modern one. So, too, his excessive systematic and descriptive precision, united as it was with comparative inattention to other than superficial characters, established a tendency, even yet not extinct, to rest contented with mere method and nomenclature instead of aiming at complete morphological knowledge.

While the artificial system was at the zenith of its fame and usefulness, Bernard de Jussieu was arranging his garden on the lines afforded by the fragmentary natural system of Linnaeus. His ideas were elaborated by his nephew and successor Antoine de Jussieu, who for the first time published diagnoses of the natural orders, so giving the system its modern character. Its subsequent elaboration and definite establishment are due mainly to the labours of Pyrame de Candolle and Robert Brown. The former concentrated his own long life and that of his son upon a new “systema naturae,” the colossal Prodromus systematis naturalis (20 vols., 1818-1873), in which 80,000 species were described and arranged. Meanwhile the penetrative genius of Brown enabled him to unravel such structural complexities as those of Conifers and Cycads, Orchids and Proteaceae, thus demonstrating the possibility of ascertaining the systematic position of even the most highly modified floral types. Both Candolle and Brown were thus no mere systematists, but genuine morphologists of the modern school. The former, as we shall afterwards see, established the theory of floral symmetry on grounds of pure comparative anatomy, and distinguished with greater success than hitherto between fundamental unity of structural type and mere superficial similarity of physiological adaptation. The latter (Humboldt’s “facile princeps botanicorum ”), using the same ideas with even keener insight, made many memorable anatomical researches, such as those on the structure of the ovule and the seed, and indeed by his demonstration of the affinities of the gymnosperms almost anticipated the discoveries of Hofmeister, who stands pre-eminent among his modern successors on account of his elucidation of the secret of phanerogamic reproduction.

The labours of Bernard and Antoine de Jussieu initiated too a vast parallel advance in zoology, the joint memoir on the classification of mammals with which Cuvier and Geoffroy St-Hilaire almost commenced their career receiving its dominant impulse from the “genera” of Antoine. Cuvier’s works correspond in zoology to those of the whole period from the Jussieus to Brown, and epitomize the results of that line of advance. Although in some respects preceded by Haller and Hunter, who compared, though mainly with physiological aim, the same parts in different organisms, and much more distinctly by Vicq d’Azyr, the only real comparative anatomist of the 18th century, he truly opens the era of detailed anatomical research united with exact comparison and clear generalization. The Règne Animal (1817) and the theory of types (vertebrate, molluscan, articulate, and radiate) are the results of this union of analysis and synthesis (although he himself, exasperated by the aberrations of the Naturphilosophie, was accustomed to proclaim the importance of detailed empiricism alone), and mark the reconstitution of taxonomy on a new basis, henceforth to be no longer a matter of superficial description and nomenclature but a complete expression of structural resemblances and differences. More even than Linnaeus he is the founder of a great school, whose names and labours are imperishable. In Germany, Bojanus, Meckel, Von Siebold, and the illustrious Johannes Müller, with his many living pupils, have carried on the work; in France, too, a succession of brilliant anatomists, such as De Quatrefages, Milne-Edwards, and Lacaze-Duthiers, are his intellectual heirs; and in England he has been admirably represented by Owen.

The histological movement inaugurated by Bichat will be subsequently discussed; the rise of embryology, how- <ever, may be briefly noted, especially since it supplied the most obvious deficiency of the Cuvierian school. Here the principal figure is Von Baer, who established independently the four types of Cuvier on developmental grounds, so for the first time applying embryology as the touchstone of anatomical classifications, besides establishing his famous law of differentiation from a general towards a special form.

It is now necessary to return to Linnaeus, whose more speculative writings contain, though encumbered by fantastic hypotheses, the idea of floral metamorphosis (“Principium florum et foliorum idem est,” &c.). About the same time, and quite independently, C. F. Wolff, the embryologist, stated the same theory with greater clearness, for the first time distinctly reducing the plant to an axis bearing appendages—the vegetative leaves—which become metamorphosed into bud-scales or floral parts through diminution of vegetative force. Thirty years later the same view was again independently developed by Goethe in his now well-known pamphlet (Versuch die Metamorphose der Pflanzen zu erklären, Gotha, 1790). In this brilliant essay the doctrine of the fundamental unity of floral and foliar parts is clearly enunciated, and supported by arguments from anatomy, development, and teratology. All the •organs of a plant are thus modifications of one fundamental organ—the leaf—and all plants are in like manner to be viewed as modifications of a common type—the Urpflanze. The controversy as to the merits and import- -ance of this essay, and of Goethe’s morphological work in .general, can scarcely be entered upon here. That Goethe discerned and proclaimed, and that more clearly than any of his predecessors or contemporaries, the fundamental idea of all morphology—the unity which underlies the multifarious varieties of organic form—and that he systematically applied this idea to the interpretation of the most important, most complex, and most varied animal and vegetable -structures, is unquestionable. The difficulties arise when we .seek to estimate the importance of his works in the chain of progress, and when we inquire whether, as some historians hold, his “urpflanze” was a mere ideal archetype, bringing forth as its fruit the innumerable metaphysical abstractions •of the Naturphilosophie, and leading his countrymen, to their fall, into all the extravagances of that system; or whether, as Haeckel maintains, it represented a concrete an- •cestral form, so anticipating the view of modern evolutionists. That to him Schelling was largely indebted for the foundation upon which he erected his philosophic edifice, as also that Goethe largely shared the same ideas, is unquestion- .able; but it must be remembered that he lived and made progress for forty years after the publication of this essay, that he was familiar with the whole scientific movement, ÷and warmly sympathized with the evolutionary views of Lamarck and Geoffroy St-Hilaire; it is not therefore to be wondered at that his writings should furnish evidence in favour of each and every interpretation of them. His Other morphological labours must not be forgotten. Independently of Vicq d’Azyr, he discovered the human premaxillary bone; independently of Oken, he proposed the vertebral theory of the skull; and before Savigny, he dis- ∙cerned that the jaws of insects were the limbs of the head.

In 1813 A. P. de Candolle published his Théorie Élémentaire de la Botanique, which he developed into the classic Organographie Végétale (1827). Although at first unacquainted with Goethe’s essay, and not clearly discerning the homology of leaves and floral parts, he established his theory of symmetry, reducing all flowers to “symmetrical” groupings of appendages on an axis and accounting for their various forms by cohesion and adhesion, by arrested or excessive development. The next great advance was the investigation by Schimper and Braun of phyllotaxis— the ascending spiral arrangement of foliar and floral organs— thus further demonstrating their essential unity.

The term morphology was first introduced by Goethe in 1817, in a subsequent essay (Zur Naturwissenschaft überhaupt, besonders zur Morphologie). It did not come into use in botany until its popularization by Auguste de St-Hilaire in his admirable Morphologie Végétale (1841), and in zoology until later, although De Blainville, who also first employed the term type, had treated the external forms of animals under “morphologie.” Though the Naturphilosophie of Schelling and its countless modifications by his followers, its mystic theories of “polarization” and the like, its apparatus of assumption and abstraction, hypothesis and metaphor, cannot here be discussed, its undoubted services must not be forgotten, since it not only stimulated innumerable reflective minds to the earnest study of natural science, but, by its incessant proclamation of the unity of nature and the free use of Platonic archetypes, gave a most powerful impulse to the study of comparative anatomy, and nobly vindicated the claims of philosophic synthesis over those of merely analytic empiricism. Among its many adherents, some are of more distinctly theological type, others metaphysical, others mystical or poetic, others, again, more especially scientific; but its most typical and picturesque figure is Lorenz Oken, who epitomizes alike the best and the worst features of the school, and among whose innumerable pseudo-morphological dreams there occasionally occurred suggestions of the greatest fruitfulness,—notably, for instance, the independent statement of the vertebral theory of the skull.

Although Lamarck shared in this movement, his great work (the Philosophie Zoologique, 1809), being aetiological rather than morphological, scarcely claims discussion here. By far the most distinguished anatomist of the transcendental school is Geoffroy St-Hilaire, who being comparatively free from the extravagances of Oken, and uniting a depth of morphological insight scarcely inferior to that of Goethe with greater knowledge of facts and far wider influence and reputation in the scientific world (which affected to sneer at the poet as necessarily a mere amateur), had enormously greater influence on the progress of science than either. He started from the same studies of anatomical detail as Cuvier, but, profoundly influenced by Buffon’s view of unity of plan and by the evolutionary doctrines of Lamarck, he rapidly diverged into new lines, and again reached that idea of serial homology of which we have so frequently noted the independent origin. His greatest work, the Philosophie Anatomique (1818-1823), contains his principal doctrines. These are—(1) the theory of unity of organic composition, identical in spirit with that of Goethe; (2) the theory of analogues, according to which the same parts, differing only in form and in degree of development, should occur in all animals; (3) the “principe des connexions,” by which similar parts occur everywhere in similar relative positions; and (4) the “principe du balancement des organes,” upon which he founded the study of teratology, and according to which the high development of one organ is allied to diminution of another. The advance in morphological theory is here obvious; unfortunately, however, in eager pursuit of often deceptive homologies, he wandered into the transcendentalism of the Naturphilosophie, and seems utterly to have failed to appreciate either the type theory of Cuvier or the discoveries of Von Baer. He earnestly defended Buffon’s and Bonnet’s earlier view of unity of plan in nature; and the controversy reached its climax in 1830, when he maintained the unity of structure in Cephalopods and Vertebrates against Cuvier before the Academy of Sciences. On the point of fact he was of course utterly defeated; the type theory was thenceforward fully accepted and the Naturphilosophie received its deathblow, while a “second empiric period” of exact anatomical and embryological research seemed for ever to replace it. Such was the popular view; only a few, like the aged Goethe, whose last literary effort was a masterly critique of the controversy, discerned that the very reverse interpretation was the deeper and essential one, that a veritable “scientific revolution” was in progress, and that the supremacy of homological and synthetic over descriptive and analytic studies was thenceforward assured. The irreconcilable feud between the two leaders really involved a reconciliation for their followers; theories of homological anatomy had thenceforward to be strictly subjected to anatomical and embryological verification, while anatomy and embryology acquired a homological aim. This union of the solid matter and rigorous method of Cuvier with the generalizing spirit and philosophic aims of Geoffroy is well illustrated in the works of Owen; and, in short, the so-called Cuvierian school is in reality thenceforward also Geoffroyan.

The further evolution of the idea of homology is sketched below (§ 7), while the extent and rapidity of the subsequent progress of the knowledge of all the structural aspects of plants and animals alike make an historical survey impossible up to the appearance of the Origin of Species (1859); however, no further qualitative advance was possible, since, as Sachs has best pointed out, morphology necessarily contains, under the Linnaean dogma of the constancy of species, the same two inconsistent and irreconcilable lines of thought which we saw represented by Caesalpinus and the early German botanists respectively,—on one side the want of strictly scientific classification, and on the other the vaguely-felt existence of a natural relationship. Strict classification of forms supposed constant excludes in fact any natural relationship. The type theory, the theory of unity of organic composition, and the like, are susceptible indeed of two explanations—they may be regarded as either expressing a creative plan, or taken as purely Platonic and archetypal ideas. Both are tenable on theological and metaphysical grounds respectively, but the fact must not be disguised that of this unity of type no explanation in the least degree scientific, i.e., in terms of the phenomena of the natural world, does or can exist. The needful solution was effected by Darwin. The “urpflanze” of Goethe, the types of Cuvier, and the like, at once became intelligible as schematic representations of ancestral organisms, which, in various and varying environments, have undergone differentiation into the vast multitude of existing forms. All the enigmas of structure become resolved; “representative” and “aberrant,” “progressive” and “degraded,” “synthetic” and “isolated,” “persistent” and “prophetic” types no longer baffle comprehension; conformity to type represented by differentiated or rudimentary organs in one organism is no longer contradicted by their entire disappearance in its near allies, while systematist and morphologist become related simply as specialist and generalizer, all through this escape from the Linnaean dogma of the fixity of species. The phenomena of individual development receive interpretation in terms of ancestral history; and embryology...