Thus we have three sources of information for the study of animal
genealogy. First, the comparative anatomy of all the different
groups of animals; second, their comparative embryology; and third,
their palaeontological history. Each source has its difficulties or
defects. But taken all together they give us a genealogical tree
which is in the main points correct, though here and there very
defective and doubtful in detail. The points in which we are left
most in doubt in regard to each ancestor are its modes of life and
locomotion, and body form. But these may temporarily vary
considerably without affecting to any great extent the general plan
of structure and the line of development of the most important
deep-seated organs.
I have chosen a line composed of forms taken from the comparative
anatomical series. All such present existing forms have probably
been modified during the lapse of ages. But I shall try to tell you
when they have diverged noticeably from the structure of the
primitive ancestor of the corresponding stage. It is much safer for
us to study concrete, actual forms than imaginary ones, however real
may have been the former existence of the latter. And, after all,
their lateral divergence is of small account compared with the great
upward and onward march of life, to the right and left of which they
have remained stationary or retrograded somewhat, like the tribes
which remained on the other side of Jordan and never entered the
Promised Land.
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