Although the separation of this order from the rest must
have taken place very early, it cannot well have taken place until
millions of animals had already come into existence. The prodigality of
nature in multiplying animal life is fully acknowledged by Darwin, and
that prodigality is apparently greatest in the lowest and most formless
type of animal. There being, then, these many millions of living
creatures in existence, the external surroundings introduce into them
many variations, and among these the special variations to which the
vertebrate type is due. It is quite clear that wherever the external
surroundings were the same or nearly the same, the variations introduced
would be the same or nearly the same. Now, it is far more probable that
external surroundings should be the same or nearly the same in many
places than that each spot should be absolutely unlike every other spot
in these particulars. The beginnings of the vertebrate order would show
themselves simultaneously, or at any rate independently, in many places
wherever external conditions were sufficiently similar. And the unity of
the plan in the vertebrata would be due, not to absolute unity of
ancestry, but to unity of external conditions at a particular epoch in
the descent of life.
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