MODERN SCIENTIFIC THOUGHT 91 



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 pre- 

 ceding 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 control- 

 ling potentialities that had no objective existence, Wolff achieved the 

 same differentiation of the homogeneous germ by means of a vis essen- 

 tialis, that sent him sailing also through the airy altitudes of final 

 causation. 



Contrary to the belief of Wolff, Bonnet and Haller found it impos- 

 sible, 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 compli- 

 cated 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 Aris- 

 totelian 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 pre- 

 formed in the germ, all the offspring of that individual must be pre- 

 formed 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 him- 

 self contributed one of the most telling facts against it when he 

 described the formation of the tubular gut of the chick by the folding 



