EVOLUTION AND EPIGENESIS. 22 1 



How far is post-fonnation to be explained as the result of pre- 

 formation, and how far as the result of external influences ? 

 That is a very different thing from the old dispute as to 

 whether there was any such thing as generation. That con- 

 tention has been settled beyond recall, and the deeper prob- 

 lems involved in generation now engage attention. The 

 question does not now turn on either of the old hinges, but 

 on what factors determine the type of development. Instead 

 of asking, are all the parts predelineated } we ask, how are 

 they delineated .^ Instead of referring development to a dens 

 ex machind, or accident, we ask, what is the mechanism of the 

 germ which enables it under suitable conditions to grow, 

 divide, differentiate, and reproduce all the complicated details 

 of its own species .-* We see that every form presented in 

 development issues as the product of what has gone before and 

 as the foundation of what is yet to come. Retrospectively, it 

 is a "determinate," prospectively, it is a "determinant." It is 

 at once consequent and antecedent, reflecting something pre- 

 existent and anticipating something post-existent. In one direc- 

 tion it illustrates the axiom ex niJiilo nil fit ; in the other, the 

 axiom, nil fit ad niliihmi. Whether we search in this direction 

 or that, and whether within or without, it is to catch the causal 

 relations of the phenomena. We may differ as to what the 

 determinants are and where they are, but all agree that they 

 are to be found out as nearly as possible. 



Now, what is our chief difference in this regard } It seems 

 to be, that some look for the determinants mainly within the 

 germ, while others search for them mainly in external influ- 

 ences. No one identifies determinants with future organs. 

 Both sides maintain that the organs of the developed form 

 have to be made, and that they must be made in orderly suc- 

 cession, as epigenesis affirms. Both sides recognize the germ 

 as something determined, and as determining something. 

 Both sides claim ultimate units of organization within the 

 germ, and both agree that external influences are responsible, 

 to some degree, for what results. 



Our difference, then, is not one of mutual contradictions, 

 each excluding the other, but one of mutual concessions, 



