Again, as will be
more fully explained in a future chapter, any difference of habits or of
haunts usually leads to some modification of colour or marking, as a
means of concealment from enemies; and there is reason to believe that
this difference will be intensified by natural selection as a means of
identification and recognition by members of the same variety or
incipient species. It has also been observed that each differently
coloured variety of wild animals, or of domesticated animals which have
run wild, keep together, and refuse to pair with individuals of the
other colours; and this must of itself act to keep the races separate as
completely as physical isolation.
_On the Advance of Organisation by Natural Selection._
As natural selection acts solely by the preservation of useful
variations, or those which are beneficial to the organism under the
conditions to which it is exposed, the result must necessarily be that
each species or group tends to become more and more improved in relation
to its conditions. Hence we should expect that the larger groups in each
class of animals and plants--those which have persisted and have been
abundant throughout geological ages--would, almost necessarily, have
arrived at a high degree of organisation, both physical and mental.
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