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Natural selection will then
begin to act in adapting the isolated portion to its new conditions, and
will do this the more quickly and the more effectually because of the
isolation. We have, however, seen reason to believe that geographical or
local isolation is by no means essential to the differentiation of
species, because the same result is brought about by the incipient
species acquiring different habits or frequenting a different station;
and also by the fact that different varieties of the same species are
known to prefer to pair with their like, and thus to bring about a
physiological isolation of the most effective kind. This part of the
subject will be again referred to when the very difficult problems
presented by hybridity are discussed.[50]

_Cases in which Isolation is Ineffective._
One objection to the views of those who, like Mr. Gulick, believe
isolation itself to be a cause of modification of species deserves
attention, namely, the entire absence of change where, if this were a
_vera causa_, we should expect to find it.


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