The great and striking influence which barriers of every
kind have had on distribution, is intelligible only on the view that
the great majority of species have been produced on one side alone,
and have not been able to migrate to the other side. Some few
families, many sub-families, very many genera, and a still greater
number of sections of genera are confined to a single region; and it
has been observed by several naturalists, that the most natural
genera, or those genera in which the species are most closely related
to each other, are generally local, or confined to one area. What a
strange anomaly it would be, if, when coming one step lower in the
series, to the individuals of the same species, a directly opposite
rule prevailed; and species were not local, but had been produced in
two or more distinct areas!
Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers of
migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we cannot
explain how the same species could have passed from one point to the
other. But the geographical and climatal changes, which have certainly
occurred within recent geological times, must have interrupted or
rendered discontinuous the formerly continuous range of many species.
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