The Theory of Evolution ve 
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 fat-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 isa 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 
