middle of the eighteenth century, there had still been no advance upon Aristotle, but there had developed a sharp contrast between two theories of development. On the one hand, Wolff supported the Aristotelian theory—now dubbed, since Harvey, epigenesis. On the other, Charles Bonnet, Albrecht von Haller and others elaborated its direct opposite in their theory of preformation.
Again, in Wolff's restatement of it, epigenesis takes on a modern aspect. The parts follow each other in development, and each part is primarily an effect of another preceding part and thereupon becomes the cause of another part that succeeds it. This is essentially the modern doctrine that one stage of development is conditioned by the stage preceding it as it conditions the stage that follows. It is crowded with suggestions; that bear no fruit, however, for lack of knowledge, in Wolff's imagination. Just as Aristotle endowed the simple germ with controlling potentialities that had no objective existence, Wolff achieved the same differentiation of the homogeneous germ by means of a vis essentialis, that sent him sailing also through the airy altitudes of final causation.
Contrary to the belief of Wolff, Bonnet and Haller found it impossible, on philosophical grounds, to conceive the beginning of the parts of an individual. For them, the germ contained the whole preformed in every part. While Bonnet insisted that man's body was not made like a watch, of added parts, but existed from the beginning as a whole, Haller was emphasizing the absurdity of believing that such a complicated apparatus as the eye could be formed as the epigenesis of the day demanded, out of crude materials by mechanical forces. Malebranche brought forward the clever device of infinite divisibility to overcome the patent objection that ordinarily the parts, whether present or not in the germ, could not at first be seen. And Bonnet admitted the obvious qualification that the parts need not exist in just the same form in the germ as they possessed in the adult. For him they belonged in the germ to a sort of invisible meshwork.
To this theory of development which sought to substitute for Aristotelian entelechies and Wolffian essential forces the conception that differentiation merely consisted in the expansion, with a push here and a pull there, of a structurally preexisting whole, numerous objections arose both in logic and in objective fact. If an individual were preformed in the germ, all the offspring of that individual must be preformed in it also. Which meant that, encased in the body of Mother Eve, one within the other, were all the germs of all the individuals of possible future generations—a sufficiently grotesque result. Wolff himself contributed one of the most telling facts against it when he described the formation of the tubular gut of the chick by the folding
