EVOLUT1O AND EPIGENESIS 415 



within the germ- cells ? how do they gradually find expression in 

 development ? what is the nature of the compelling necessity 

 that mints and coins the chick out of a drop of living matter ? 

 what is the regulative principle that secures the order and 

 progress which, by devious and often circuitous paths, results 

 in the fully-formed organism ? 



The solution is still far off, and perhaps we shall never get 

 beyond saying that a germ-cell has the power of developing, 

 just as a crystal has the power of growing. But this need not 

 hinder us from trying imaginatively to formulate what takes 

 place, for it is largely through these provisional hypotheses that 

 research is provoked and facts are won. 



It may be said that there are two main ways of considering 

 the fundamental problem of " individual becoming " which 

 embryology raises, and as these are analogous to the theories 

 of " Epigenesis " and " Evolutio " which were so much dis- 

 cussed in the seventeenth and eighteenth centuries, the same 

 catch-words may be retained. 



The Old Evolutio and Epigenesis. Without going into 

 the details of an often-repeated story, we may recall how 

 men, like Bonnet (1720-93) and Haller (1708-77), maintained 

 the preformation of the organism and all its parts within the 

 germ. The egg, Bonnet said, contained tres en petit the ele- 

 ments of all the organic parts. " Es gibt kein Werden," Haller 

 said ("There is no becoming"). Those of this preformationist 

 school regarded the apparent new formation of organs during 

 development as an illusion ; what occurs is only an unfolding 

 (evolutio) of a preformed miniature. How the germ came to have 

 this preformed miniature, they could not tell. 



On the other hand, Caspar Friedrich Wolff (1733-94) was 

 the pioneer of another school, in maintaining the reality of 

 what he saw a gradual differentiation from apparent simplicity 

 to obvious complexity. The various organs of the developing 

 embryo make their appearance successively and gradually, 



