of range by adverse conditions may act in one direction only,
and over a limited district, so as ultimately to divide the specific area
into two separated parts, in each of which a portion of the species will
continue to maintain itself. We have seen that there is reason to believe
that this has occurred in a very few cases both in North America and in
Northern Asia. (_See_ pp. 65-68.) But the same thing has certainly occurred
in a considerable number of cases, only it has resulted in the divided
areas being occupied by _representative forms_ instead of by the very same
species. The cause of this is very easy to understand. We have already
shown that there is a large amount of local variation in a considerable
number of species, and we may be sure that were it not for the constant
intermingling and intercrossing of the individuals inhabiting adjacent
localities this tendency to local variation in adaptation to slightly
different conditions, would soon form distinct races. But as soon as the
area is divided into two portions the intercrossing is stopped, and the
usual result is that two closely allied races, classed as representative
species, become formed. Such pairs of allied species on the two sides of a
continent, or in two detached areas, are very numerous; and their existence
is only explicable on the supposition that they are descendants of a parent
form which once occupied an area comprising that of both of them,--that
this area then became discontinuous,--and, lastly, that, as a consequence
of the discontinuity, the two sections of the parent species became
segregated into distinct races or new species. {410}

Now, when the division of the area leaves one portion of the species in an
island, a similar modification of the species, either in the island or in
the continent, occurs, resulting in closely-allied but distinct forms; and
such forms are, as we have seen, highly characteristic of island-faunas.
But islands also favour the occasional preservation of the unchanged
species--a phenomenon which very rarely occurs in continents. This is
probably due to the absence of competition in islands, so that the parent
species there maintains itself unchanged, while the continental portion, by
the force of that competition, is driven back to some remote mountain area,
where it also obtains a comparative freedom from competition. Thus may be
explained the curious fact, that the species common to Formosa and India
are genera