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 ancestral 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.
In the light of the preceding discussion concerning the evidence in favor of the transmutation theory, we may now proceed to sum up our general conclusions, and at the same time discuss some further possibilities in regard to the descent theory.
The most widely accepted view in regard to the theory of organic evolution is that which looks upon the resemblances between the members of a group as due to their common descent from one original species that has broken up, as it were, into a number of new forms. Strictly applied, this means that all the vertebrates have come from one original species, all the mollusks from another, the echinoderms from a third, etc. Even farther back there may have been a common ancestral species for any two of the large groups, as, for example, the annelids and the mollusks; and if the relationship of all the many-celled forms be looked upon as probable, then they too have originated from one ancestral species.
Many zoologists appear to hesitate to apply strictly this fundamental idea contained in the transmutation theory, because, perhaps, they feel that it does not fit in with their general experience of living forms. Yet there can be no doubt that it is the primary conception of the transmutation theory. This is, however, not the whole question, for we must further consider the number of individuals of a species that are involved.