The Theory of Evolution 83 



maintains is a repetition of the other, but only in form, not in 

 actual contents. And in another connection we are told that 

 the cause of this repetition is that the gastrula is the simplest 

 way in which the later stages can be reached, and, therefore, 

 it has been retained. It seems to me that Hertwig has under- 

 taken an unnecessary and impossible task when he attempts 

 to adjust the old recapitulation theory to more modern 

 standards. His statement that the egg is entirely different 

 from its amoeba prototype is, of course, only the view generally 

 held by all embryologists. His mystical statement that the 

 embryonic form repeats the ancestral adult stage in its form., 

 but not in its contents, will scarcely recommend itself as a 

 model of clear thinking. Can we be asked to believe for 

 instance that a young chick repeats the ancestral adult fish 

 form but not the contents of the fish ? 



In conclusion, then, it seems to me that the idea that adult 

 ancestral stages have been pushed back into the embryo, and 

 that the embryo recapitulates in part these ancestral adult 

 stages is in principle false. The resemblance between the 

 embryos of higher forms and the adults of lower forms is 

 due, as I have tried to show, to the presence in the embryos 

 of the lower groups of certain organs that remain in the 

 adult forms of this group. It is only the embryonic stages of 

 the two groups that we are justified in comparing ; and their 

 resemblances are explained on the assumption that there 

 has been an ancestral adult form having these embryonic 

 stages in its development and these stages have been handed 

 down to the divergent lines of its descendants. 



Since, we have come to associate with the name of the 

 recapitulation theory the idea of the recurrence of an ances- 

 tral adult form, it may be better to find a substitute for this 

 term. I suggest, therefore, for the view, that the embryos 

 of the higher group repeat the modified form of the embryos 

 of the lower groups, the term, the theory of embryonic 

 repetition, or, more briefly, the repetition theory. 



