It will
be seen that evolution not only introduces a lucid order and
arrangement into our thousands of species of living and fossil
mammals, but throws an admirable light on the higher animal world
of our time. The various orders into which the zoologist puts our
mammals are seen to be the branches of a living tree, approaching
more and more closely to each other in early Tertiary times, in
spite of the imperfectness of the geological record. We at last
trace these diverging lines to a few very primitive, generalised,
patriarchal groups, which in turn approach each other very
closely in structure, and plainly suggest a common Cretaceous
ancestor. Whether that common ancestor was an Edentate, an
Insectivore, or Creodont, or something more primitive than them
all, is disputed. But the divergence of nearly all the lines of
our mammal world from those patriarchal types is admirably clear.
In the mutual struggle of carnivore and herbivore, in adaptation
to a hundred different environments (the water, the land, and the
air, the tree, the open plain, the underground, the marsh, etc.)
and forms of diet, we find the descendants of these patriarchal
animals gradually developing their distinctive characters.
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