in number altogether) that they may all be comprised in from eight to
twelve great groups. But even in these groups, the stem-forms or
animal types, certain organs (especially the alimentary canal) can be
proved to have been originally the same for all. We can only explain
by the theory of evolution this essential unity in internal structure
of all these animal forms that differ so much in outward appearance.
This wonderful fact can only be really understood and explained when
we regard the internal resemblance as an inheritance from common-stem
forms, and the external differences as the effect of adaptation to
different environments.

In recognising this, comparative anatomy has itself advanced to a
higher stage. Gegenbaur, the most distinguished of recent students of
this science, says that with the theory of evolution a new period
began in comparative anatomy, and that the theory in turn found a
touch stone in the science. "Up to now there is no fact in comparative
anatomy that is inconsistent with the theory of evolution; indeed,
they all lead to it. In this way the theory receives back from the
science all the service it rendered to its method." Until then
students had marvelled at the wonderful resemblance of living things
in their inner structure without being able to explain it. We are now
in a position to explain the causes of this, by showing that this
remarkable agreement is the necessary consequence of the inheriting of
common stem-forms; while the striking difference in outward appearance
is a result of adaptation to changes of environment. Heredity and
adaptation alone furnish the true explanation.

But one special part of comparative anatomy is of supreme interest and
of the utmost philosophic importance in this connection. This is the
science of rudimentary or useless organs; I have given it the name of