Here, however, we cannot yet chronicle any such thoroughgoing revolution of general conceptions; the basis of detailed empirical knowledge was not nearly broad enough for that, and it was in the acquiring of such a foundation that the next three centuries, from the sixteenth to the end of the eighteenth, were eagerly occupied.
The first step necessary was to collate the items of individual knowledge in regard to the various forms of life, and to bring the whole in unified form into general notice. This need was met for the first time by Conrad Gessner's Thierbuch, a handsome folio volume, printed at Zurich in 1551, and embellished with numerous woodcuts, some of them very good. This was followed, in 1600, by a great work in many volumes, written in Latin, by a professor of Bologna, Aldrovandi. Not native animals alone but foreign ones also were described in these works, for, after the discovery of America and the opening up of communication with the East Indies, many new animal and plant forms came to the knowledge of European nations by way of the sea. Thus Francesco Hernandez (died 1600), physician in ordinary to Philip II, described no fewer than forty new Mammals, more than two hundred Birds, and many other American animals.
Again, in a quite different way, the naturalist's field of vision was widened, namely, by the invention of the simple microscope, with which Leeuwenhoek first discovered the new world of Infusorians, and Swammerdam made his notable observations on the structure and development of the very varied minute animal inhabitants of fresh water. In the same century, the seventeenth, anatomists like Tulpius, Malpighi, and many others extended the knowledge of the internal structure of the higher animals and of Man, and a foundation was laid for a deeper insight into the nature of vital functions by the discovery of the circulation of the blood in Man and the higher animals. In the following century, the eighteenth, this path of active research was eagerly followed, and we need only mention such names as Réaumur, Rösel von Rosenhof, De Geer, Bonnet, J. Chr. Schäfer, and Ledermüller, to be immediately reminded of the wealth of facts about the structure, life, and especially the development of our indigenous animals, which we owe to the labours of these men.
All these advances, great and many-sided as they were, did not at once lead to a renewal of the attempt of Empedocles to explain the origin of the organic world. This was as yet not even recognized as a problem requiring investigation, for men were content to take the world of life simply as a fact. The idea of getting beyond the naïve, poetic standpoint of the Mosaic story of Creation was as yet remote from the minds of naturalists, partly because they were wholly fascinated by the observation of masses of details, but chiefly because, first by the English physician, John Ray (died 1678), then by the great Swede, Carl Linné, the conception of organic 'species' had been formulated and sharply defined. It is true enough that before the works of these two men 'species' had been spoken of, but without being connected with any definite idea; the word was used rather in the same vague sense as the word 'genus,' to designate one of the smaller groups of organic forms, but without implying any clear idea of its scope or of its limitations. Now, however, for the first time, the term 'species' came to be used strictly to mean the smallest homogeneous group of individual forms of life upon the earth. John Ray held that the surest indication of a 'species' was that its members had been produced from the same seed; that is, 'forms which are of different species maintain this specific nature constantly, and one species does not arise from the seed of another.' Here we have the germ of the doctrine of the absolute nature and the immutability of species which Linné briefly characterized in these words: 'Species tot sunt, quot formæ ab initio creatæ sunt,' 'there are just so many species as there were forms created in the beginning.' It is here clearly implied, that species as we know them have been as they are from all time, that, therefore, they exist in nature as such and unchangeably, and have not been merely read into nature by man.
This view, though we cannot now regard it as correct, was undoubtedly reasonable, and thoroughly in accordance with the spirit of the time; it was congruent with the knowledge, and above all with the scientific endeavours of the age. In the eighteenth century there was danger that all outlook on nature as a whole would be lost—smothered under the enormous mass of isolated facts, and especially under the inundation of diverse animal and plant forms which were continually being recognized. It must therefore have been regarded as a real deliverance, when Linné reduced this chaos of forms to a clearly ordered system, and relegated each form to its proper place and value in relation to the whole. How, indeed, could the great systematist have performed his task at all, if he had not been able to work with definite and sharply circumscribed groups of forms, if he had not been able to regard at least the lowest elements of his system, the species, as fixed and definite types? On the other hand, Linné was much too shrewd an observer not to entertain, in the course of his long life, and under the influence of the continually accumulating material, doubts as to the correctness of his assumption of the fixity and absoluteness of his species. He discovered from his own experience, what is fully borne out by ours, that it is easy enough to define a species when there are only a few specimens of a form to deal with, but that the difficulty increases in proportion to the number and to the diversity of habitat of those that are to be brought under one category. In the last edition of the Systema Naturæ we find very noteworthy passages in which Linné wonders whether, after all, a species may not change, and in the course of time diverge into varieties, and so forth. Of these doubts no notice was taken at the time; the accepted doctrine of the fixity of species was held to and even raised to the rank of a scientific dogma. Georges Cuvier, the great disciple of the Stuttgart 'Karlschule,' accentuated the doctrine still further by his establishment of animal-types, the largest groups of forms in the animal kingdom within which a definite and fundamentally distinct plan of architecture prevails. His four types, Vertebrates, Molluscs, Articulate and Radiate animals, furnished a further corroboration of the absolute nature of species, since they seemed to show that even the highest and most comprehensive groups are sharply defined off from one another.
Let me add that this doctrine of the absolute nature of species was not fully elaborated till our own day, when the Swiss (afterwards American) naturalist, Louis Agassiz, went so far as to maintain that not only the highest and the lowest categories, but all those coming between them, were categories established and sharply separated by Nature herself. But in spite of much ingenuity and his wide and comprehensive outlook he exerted himself in vain to find satisfactory and really characteristic definitions of what was to be considered a class, an order, a family, or a genus. He did not succeed in finding a rational definition of these systematic concepts, and his endeavour may be regarded as the last important attempt to prop up an interpretation of nature already doomed to fall. But in referring to Louis Agassiz I have anticipated the historical course of scientific development, and must therefore go back to the last quarter of the eighteenth century.
The first unmistakable pioneer of the theory of descent, which now emerged for the first time as a scientific doctrine, was our great poet Goethe. He has indeed been often named as the founder of the theory, but that seems to me saying too much. It is true, however, that the inquiring mind of the poet certainly recognized in the structure of 'related' animals the marvellous general resemblances amid all the differences in detail, and he probed for the reason of these form-relations. Through the science of 'comparative anatomy,' as it was taught at the close of the century by Kielmeyer, Cuvier's teacher, and later by Cuvier himself, Blumenbach, and others, numerous facts had become known, which paved the way for such questions. It had, for instance, been recognized that the arm of man, the wing of the bird, the paddle of the seal, and even the foreleg of the horse, contain essentially the same chain of bones, and Goethe had already expressed these relations in his well-known verse,
'Alle Gestalten sind ähnlich, doch keine gleichet der andern,
Und so deutet der Chor auf ein geheimes Gesetz.'