EVOLUTION 749 tive study, but they fell upon evil times. The vast authority of Cuvier was employed in support of the traditionally respectable hypotheses of special creation and of catastro- phism ; and the wild speculations of the Discours sur les Jievolutions de la Surface du Globe were held to be models of sound scientific thinking, while the really much more sober and philosophical hypotheses of the Hydroyeologie were scouted. For many years it was the fashion to speak of Lamarck with ridicule, while Treviranus was altogether ignored. Nevertheless, the work had been done. The conception of evolution was henceforward irrepressible, and it incessantly reappears, in one shape or another, 1 up to the year 1858, when Mr Darwin and Mr Wallace published their Theory of Natural Selection. The Origin of Species appeared in 1859; and it is within the knowledge of all whose memories go back to that time, that, henceforward, the doctrine of evolu tion has assumed a position and acquired an importance which it never before possessed. In the Origin of Species, and in his other numerous and important contributions to the solution of the problem of biological evolution, Mr Darwin confines himself to the discussion of the causes which have brought about the present condition of living matter, assuming such matter to have once come into existence. On the other hand, Mr Spencer 2 and Professor Haeckel 3 have dealt with the whole problem of evolution. The profound and vigorous writings of Mr Spencer embody the spirit of Descartes in the knowledge of our own day, and may be regarded as the " Principes des Philosophic " of the 19th century ; while, whatever hesitation may not unfrequently be felt by less daring minds, in following Haeckel in many of his speculations, his attempt to sys tematize the doctrine of evolution and to exhibit its in fluence as the central thought of modern biology, cannot fail to have a far-reaching influence on the progress of science. If we seek for the reason of the difference between the scientific position of the doctrine of evolution a century ago, and that which it occupies now, we shall find it in the great accumulation of facts, the several classes of .which have been enumerated above, under the second to the eighth heads. For those which are grouped under the second to the seventh of these classes, respectively, have a clear significance on the hypothesis of evolution, while they are unintelligible if that hypothesis be denied. And those of the eighth group are not only unintelligible without the assumption of evolution, but can be proved never to be discordant with that hypothesis, while, in some cases, they are exactly such as the hypothesis requires. The demonstration of these assertions would require a volume, but the general nature of the evidence on which they rest may be briefly indicated. 2. The accurate investigation of the lowest forms of animal life, commenced by Leeuwcnhoek and Swammerdam, and continued by the remarkable labours of Reaumur, Trembley, Bonnet, and a host of other observers in the latter part of the 17th and the first half of the 18th cen turies, drew the attention of biologists to the gradation in the complexity of organization which is presented by living beings, and culminated in the doctrine of the " dchelle des etres," so powerfully and clearly stated by Bonnet ; and, before him, adumbrated by Locke and by Leibnitz. In the then state of knowledge, it appeared that all the species of animals and plants could be arranged in one series; in such a manner that, by insensible gradations, the mineral passed into the plant, the plant into the polype, the polype into the worm, and so, through gradually higher forms of life, to man, at the summit of the animated world. 1 Seethe "Historical Sketch " prefixed to the last edition of the Origin of Species.
- First Principles and Principles of Biology, 1860-1864.
3 Generdle Morpiwloyie, 1866. But, as knowledge advanced, this conception ceased to be tenable in the crude form in which it was first put forward. Taking into account existing animals and plants alone, it became obvious that they fell into groups which were more or less sharply separated from one another; and, moreover, that even the species of a genus can hardly ever be arranged iu linear series. Their natural resemblances and differences are only to be expressed by disposing them as if they were branches springing from a common hypo thetical centre. Lamarck, while affirming the verbal proposition that animals form a single series, was forced by his vast acquaintance with the details of zoology to limit the asser tion to such a series as may be formed out of the abstrac tions constituted by the common characters of each group. 4 Cuvier on anatomical, and Yon Baer on embryological grounds, made the further step of proving that, even in this limited sense, animals cannot be arranged in a single series, but that there are several distinct plans of organi zation to be observed among them, no one of which, in its highest and most complicated modification, leads to any of the others. The conclusions enunciated by Cuvier and Von Baer have been confirmed in principle by all subsequent research into the structure of animals and plants. But the effect of the adoption of these conclusions has been rather to sub stitute a new metaphor for that of Bonnet than to abolish the conception expressed by it. Instead of regarding living things as capable of arrangement in one series like the steps of a ladder, the results of modern investigation com pel us to dispose them as if they were the twigs and branches of a tree. The ends of the twigs represent individuals, the smallest groups of twigs species, larger groups genera, and so on, until we arrive at the source of all these ramifica tions of the main branch, which is represented by a common plan of structure. At the present moment, it is impossible to draw up any definition, based on broad anatomical or developmental characters, by which any one of Cuvier s great groups shall be separated from all the rest. On the contrary, the lower members of each tend to converge towards the lower members of all the others, The same may be said of the vegetable world. The apparently clear distinction between flowering and flowerless plants has been broken down by the series of gradations between the two exhibited by the Lycopodiacece, Jthizocarpece, and Gymnospermece. The groups of Fungi, Lichenecc, and Ahjce have completely run into one another, and, when the lowest forms of each are alone considered, even the animal and vegetable kingdoms cease to have a definite frontier. If it is permissible to speak of the relations of living forms to one another metaphorically, the similitude chosen must undoubtedly be that of a common root, whence two main trunks, one rapresenting the vegetable and one the animal world, spring; and, each dividing into a few main branches, these subdivide into multitudes of branchlets and these into smaller groups of twigs. As Lamarck has well said 5 II n y a qne ccux qui se sont longtcmps et fortement occupe s de la determination des especes, et qui ont consulte de riches collections, qui pcuvent savoir jusqu a quel point les especes, parmi les corps vivants se fondent les unes dans les autres, et qui ont pu se con- vaincre que, dans les parties ou nous voyons des CSJKCCS isoles, cela n est ainsi que parcequ il nous en manque d autres qui en sont plus voisines et que nous n avons pas encore recueillies. Je ne veux pas dire pour cela que les animaux qui existent for- ment une serie, tres-simple et partout egaleincnt miancee; inais je dis qu ils fornieut une serie rameuse, irregulierement graduee et qui 4 " II 8 a<rit done de prouver que la serie qui constitute I echelle animale reside essentiellenient dans la distribution des masses princi- pales qui la composent et non dans oelle des especes ni meme toujourg dans celle des genres." Phil. Zonlnf/irfiie, chap. T.
- Philosophic Zoologique, premiere partie, chap. iii.
