descent of two or more species is based on the totality of such
resemblances that still remain in large part after each change has taken
place. In this sense the argument from comparative anatomy, while not a
demonstration, carries with it, I think, a high degree of probability.

_The Evidence from Embryology_

In passing from the egg to the adult the individual goes through a series
of changes. In the course of this development we see not only the
beginnings of the organs that gradually enlarge and change into those of
the adult animal, but also see that organs appear and later disappear
before the adult stage is reached. We find, moreover, that the young
sometimes resemble in a most striking way the adult stage of groups that we
place lower in the scale of evolution.

Many years before Darwin advanced his theory of evolution through natural
selection, the resemblance of the young of higher animals to the adults of
lower animals had attracted the attention of zoologists and various views,
often very naive, had been advanced to account for the resemblance. Among
these speculations there was one practically identical with that adopted by
Darwin and the post-Darwinians, namely that the higher animals repeat in
their development the _adult stages_ of lower animals. Later this view
became one of the cornerstones of the theory of organic evolution. It
reached its climax in the writings of Haeckel, and I think I may add
without exaggeration that for twenty-five years it furnished the chief
inspiration of the school of descriptive embryology. Today it is taught in
practically all textbooks of biology. Haeckel called this interpretation
the Biogenetic Law.

[Illustration: FIG. 6. Young trout (Trutta fario) six days after hatching.
(After Ziegler.)]

It was recognized, of course, that many embryonic stages could not possibly
represent ancestral animals. A young fish with a huge yolk sac attached
(fig. 6) could scarcely ever have led a happy, free life as an adult
individual. Such stages were interpreted, however, as _embryonic_ additions
to the original ancestral type. The embryo had done something on its own
account.

In some animals the young have structures that attach them to the mother,
as does the placenta of the mammals. In other cases the young develop
membranes about themselves--like the amnion of the chick (fig. 7) and
mammal--that would have shut off an adult animal from all intercourse with
the outside world.