In the latter
cases to find the common ancestor we must go back to the grandfather
or great-grandfather. The same difficulty, but greatly enhanced,
meets us when we try to make a genealogical tree of the animal
kingdom. Thus it seems altogether probable that all higher forms are
descended from an ancestor of the same general structure and grade
of organization as the turbellaria, although probably free swimming,
and hence with somewhat different form and development, especially
of the muscular system. It seems to me altogether probable that all,
except possibly Mollusca, are descended from a common ancestor
closely resembling the schematic worm last described. Some would,
however, maintain that they diverged rather earlier than even the
turbellaria; others after the schematic worm, if such ever existed.
As far as our argument is concerned it makes little difference which
of these views we adopt.
From our turbellaria, or possibly from some even more primitive
ancestor, many lines diverged. And this was to be expected. The
coelenterata, as we saw in hydra, had developed rude digestive and
reproductive systems. The higher groups of this kingdom had
developed all, or nearly all, the tissues used in building the
bodies of higher animals--muscular, reproductive, connectile,
glandular, nervous, etc.
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