The
characteristics of extinct genera of mammals exhibit everywhere
indications that their living representatives in early life resemble
them more than they do their own parents. A minute comparison of a
young elephant with any mastodon will show this most fully, not only
in the peculiarities of their teeth, but even in the proportion of
their limbs, their toes, etc. It may therefore be considered as
a general fact that the phases of development of all living
animals correspond to the order of succession of their extinct
representatives in past geological times. The above statements are
quoted almost word for word from Professor Agassiz's "Essay on
Classification." The larvae of barnacles and other more degraded
parasitic crustacea are almost exactly like those of Crustacea in
general. The embryos of birds have a long tail containing almost or
quite as many vertebrae as that of archaeopteryx. But most of these
never reach their full development but are absorbed into the pelvis,
or into the "ploughshare" bone supporting the tail feathers. Thus
older forms may be said to have retained throughout life a condition
only embryonic in their higher relatives. And the natural
classification gave the order not only of geological succession but
also of stages of embryonic development. Thus the system of
classification improved continually, although more and more
intermediate forms, like archaeopteryx, were discovered, and certain
aberrant groups could find no permanent resting-place.
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