at
their unity of construction is due to that fact; but if we go back in
thought to the time at which the special peculiarities were introduced
which really constituted the order and separated it from other animals,
we see that it is by no means clear that it originated with one pair or
with one individual, and that, on the contrary, the probabilities are
the other way. 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. Hence it follows that the separation of animals
into orders and genera and even into species took place, if not for the
most part yet very largely, at a very early period in the history of
organic evolution. Of course the descendants of any one of the original
vertebrata might, and probably in not a few cases did, branch off into
new subdivisions and yet again into further subdivisions, and we are
always justified in looking for unity of ancestry among all the species.
But it is also quite possible that any species may be regularly
descended, without branching off at all, from one of the originals, and
that other species that resemble it may owe the resemblance simply to
very great similarity of external conditions. To find, for instance, the
unity of ancestry between man and the other animals, it will certai