The Theory of Evolution 73 



phenomenon is that some sort of genetic connection must 

 exist between the different forms; and while not explicitly- 

 stated, yet there is not much doubt that some at least of 

 these authors have had in mind the view that the annelids 

 and mollusks are descended from common ancestors whose 

 eggs segmented as do those of most of the mollusks and 

 annelids of the present day. This conclusion is, I believe, of 

 more far-reaching importance than has been supposed, and 

 may furnish the key that will unlock the whole question of 

 the resemblance of embryos to supposed ancestral forms. 

 It is a most fortunate circumstance that in the case of this 

 cell lineage the facts are of such a kind as to preclude the 

 possibility that the stages in common could ever have been/ 

 ancestral adult stages. If this be granted then only two 

 interpretations are possible : the results are due either to a 

 coincidence, or to a common embryonic form that is repeated 

 in the embryo of many of the descendants. That the simi- 

 larity is not due to a coincidence is made probable from the 

 number and the complexities of the cleavage stages. 



I believe that we can extend this same interpretation to 

 all other cases of embryonic resemblance. It will explain 

 the occurrence of gill-slits in the embryo of the bird, and the 

 presence of a notochord in the higher forms in exactly the 

 same way as the cleavage stages are explained. But how, 

 it may be asked, can we explain the apparent resemblance 

 between the embryo of the higher form and the adult of 

 lower groups. The answer is that this resemblance is decep- 

 tive, and in so far as there is a resemblance it depends 

 on the resemblance of the adult of the lower form to its own 

 embryonic stages with which we can really make a compari- 

 son. The gill-slits of the embryo of the chick are to be com- 

 pared, not with those of the adult fish, but with those of the 

 embryo of the fish. It is a significant fact, in this connection, 

 that the gill-slits appear as early in the embryo of the fish as 

 they do in the bird ! The notochord of the embryo bird is 



