On the view of all the species of the same genus
having descended from a common parent, and having inherited much in common,
we can understand how it is that allied species, when placed under widely
different conditions of life, yet follow nearly the same instincts; why the
thrushes of tropical and temperate South America, for instance, line their
nests with mud like our British species. On the view of instincts having
been slowly acquired through natural selection, we need not marvel at some
instincts being not perfect and liable to mistakes, and at many instincts
causing other animals to suffer.
If species be only well-marked and permanent varieties, we can at once see
why their crossed offspring should follow the same complex laws in their
degrees and kinds of resemblance to their parents--in being absorbed into
each other by successive crosses, and in other such points--as do the
crossed offspring of acknowledged varieties. This similarity would be a
strange fact, if species had been independently created and varieties had
been produced through secondary laws.
If we admit that the geological record is imperfect to an extreme degree,
then the facts, which the record does give, strongly support the theory of
descent with modification. New species have come on the stage slowly and
at successive intervals; and the amount of change after equal intervals of
time, is widely different in different groups.
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