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 amceba 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 zdea that adult 
ancestral stages have been pushed back into the embryo, and 
that the embryo recapitulates in part these ancestral adult 
Stages ts 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. 
