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 Charles Leopold Laurillard’s Éloge, prefixed to the last edition of the Ossemens fossiles, the “radical de l’être” is much the same thing as Aristotle’s “particula genitalis” and Harvey’s “ovum.”[25]
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 cru que le cœur 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’œuf avant qu’il ait été couvé.”[26]
“J’ai ouvert une grande quantité d’œufs 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 cicatrule au moment qu’il sort du corps de la poule.”[27]
The “moule intérieur” of Buffon is the aggregate of elementary parts which constitute the individual, and is thus the equivalent of Bonnet’s germ,[28] as defined in the passage cited above. But Buffon further imagined that innumerable “molécules 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 of the “Medici” whom Harvey condemns. The “molécules 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 generatìonis, 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 19th 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 be 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 (see [Embryology]) and the evolution of the sum of living beings.
The Evolution of the Sum of Living Beings.—The notion that all the kinds of animals and plants may have come into existence by the growth and modification of primordial germs is as old as speculative thought; but the modern scientific form of the doctrine can be traced historically to the influence of several converging lines of philosophical speculation and of physical observation, none of which go further back than the 17th century. These are:—
1. The enunciation by Descartes of the conception that the physical universe, whether living or not living, is a mechanism, and that, as such, it is explicable on physical principles.