As to what this law was he did not at that time pronounce an opinion, though he may even then have thought of the transformation of species. At first he contented himself with seeking for an ideal archetype or 'Urtypus' which was supposed to lie at the foundation of a larger or smaller group. He discovered the archetypal plant or 'Urpflanze,' when he rightly recognized that the parts of the flower are nothing more than modified leaves. He spoke plainly of the 'metamorphosis of plants,' meaning by that the transformation of his 'archetype' into the endless diversity of actual plant forms. But at first he certainly thought of this transformation only in the ideal sense, and not as a factual evolutionary process.
The first who definitely maintained the latter view was, remarkably enough, the grandfather of the man who, in our own day, made the theory of descent finally triumphant, the English physician Erasmus Darwin, born 1731. This quiet thinker published, in 1794, a book entitled Zoonomia, and in it he takes the important step of substituting for Goethe's 'secret law' a real relationship of species. He proclaims the gradual establishment and ennobling of the animal world, and bases his view mainly on the numerous obvious adaptations of the structure of an organ to its use. I have not been able to find any passage in the book in which he has expressly indicated that, because many of the conditions of life could not have existed from the beginning, these adaptations are therefore, as such, an argument for the gradual transformation of species. But he assumed that such exact adaptations to the functions of an organ could only arise through the exercise of that function, and in this he saw a proof of transformation. Goethe had expressed the same idea when he said, 'Thus the eagle has conformed itself through the air to the air, the mole through the earth to the earth, and the seal through the water to the water,' and this shows that he too at one time thought of an actual transformation. But neither he nor Erasmus Darwin were at all clear as to how the use of an organ could bring about its variation and transformation. The latter only says that, for instance, the snout of the pig has become hard through its constant grubbing in the ground; the trunk of the elephant has acquired its great mobility through the perpetual use of it for all sorts of purposes; the tongue of the herbivore owes its hard, grater-like condition to the rubbing to and fro of the hard grass in the mouth, and so on. How acute and thoughtful an observer Erasmus Darwin was, is shown by the fact that he had correctly appreciated the biological significance of many of the colour-adaptations of animals to their surroundings, though it was reserved for his grandson to make this fully clear at a much later date. Thus he regarded the varied colouring of the python, of the leopard, and of the wild cat as the best adapted for concealing them from their prey amid the play of light and shadow in a leafy thicket. The black spot in front of the eye of the swan he considered an arrangement to prevent the bird from being dazzled, as would happen if that spot were as snow-white as the rest of the plumage.
At the end of the book he sums up his views in the following sentences: 'The world has been evolved, not created; it has arisen little by little from a small beginning, and has increased through the activity of the elemental forces embodied in itself, and so has rather grown than suddenly come into being at an almighty word.' 'What a sublime idea of the infinite might of the great Architect! the Cause of all causes, the Father of all fathers, the Ens entium! For if we could compare the Infinite it would surely require a greater Infinite to cause the causes of effects than to produce the effects themselves.'
In these words he sets forth his position in regard to religion, and does so in precisely the same terms as we may use to-day when we say: 'All that happens in the world depends on the forces that prevail in it, and results according to law; but where these forces and their substratum, Matter, come from, we know not, and here we have room for faith.'
I have not been able to discover whether the Zoonomia, with its revolutionary ideas, attracted much attention at the time when it appeared, but it would seem not. In any case, it was afterwards so absolutely forgotten, that in an otherwise very complete History of Zoology, published in 1872 by Victor Carus, it was not even mentioned. About a year after the appearance of Zoonomia, Isidore Geoffrey St.-Hilaire in Paris expounded the view that what are called species are really only 'degenerations,' deteriorations from one and the same type, which shows that he too had begun to have doubts as to the fixity of species. Yet it was not till the third decade of the nineteenth century that he clearly and definitely took up the position of the doctrine of transformation, and to this we shall have to return later on.
But as early as the first decade of the century this position was taken up by two noteworthy naturalists, a German and a Frenchman, Treviranus and Lamarck.
Gottfried Reinhold Treviranus, born at Bremen in 1776, an excellent observer and an ingenious investigator, published, in 1802, a book entitled Biologie, oder Philosophie der lebenden Natur [Biology, or Philosophy of Animate Nature], in which he expresses and elaborates the idea of the Evolution theory with perfect clearness. We read there, for instance: 'In every living being there exists a capacity for endless diversity of form; each possesses the power of adapting its organization to the variations of the external world, and it is this power, called into activity by cosmic changes, which has enabled the simple zoophytes of the primitive world to climb to higher and higher stages of organization, and has brought endless variety into nature.' But where the motive power lies, which brings about these transformations from the lowliest to ever higher forms of life, was a question which Treviranus apparently did not venture to discuss. To do this, and thus to take the first step towards a causal explanation of the assumed transformations, was left for his successor.
Jean Baptiste de Lamarck, born in 1744 in a village of Picardy, was first a soldier, then a botanist, and finally a zoologist. He won his scientific spurs first by his Flora of France, and zoology holds him in honour as the founder of the category of 'vertebrates.' Not that he occupied himself in particular detail with these, but he recognized the close alliance of the classes of animals in question—an alliance which was subsequently expressed by Cuvier by the systematic term 'type' or 'embranchement.'
In his Philosophie zoologique, published in 1809, Lamarck set forth a theory of evolution whose truth he attempted to vindicate by showing—as Treviranus had done before him—that the conception of species, on the immutability of which the whole hypothesis of creation had been based, was an artificial one, read into nature by us; that sharply circumscribed groups do not exist in nature at all; and that it is often very difficult, and not infrequently quite impossible, to define one species precisely from allied forms, because it is connected with these on all sides by transition stages. Groups of forms which thus melted into one another indicated that the doctrine of the fixity of species could not be correct, any more than that of their absolute nature. Species, he maintained, are not immutable, and are not so old as nature; they are fixed only for a certain time. The shortness of our life prevents our directly recognizing this. 'If we lived a much shorter time, say about a second, the hour-hand of the clock would appear to us to stand still, and even the combined observations of thirty generations would afford no decisive evidence as to the hand's movement, and yet it had been moving.'
The causes on which, according to Lamarck, the transformation of species, their modification into new species, depends, lie in the changes in the conditions of life which must have occurred ceaselessly from the earliest period of the earth's history till our own day, now here, now there, due in part to changes in climate and in food-supply, in part to changes in the earth's crust by the rising or sinking of land-masses, and so forth. These external changes have sometimes been the direct cause of changes in bodily structure, as in the case of heat or cold; but they have sometimes and much more effectively operated indirectly. Thus changed conditions may have prompted an animal of a given species to use certain parts of its body in a new way, more vigorously, or less actively, or even not at all, and the more vigorous use, or, conversely, the disuse, has brought about variations in the organ in question.