ide. 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. So that we are reduced to
consider whether the exceptions to continuity of range are so numerous
and of so grave a nature, that we ought to give up the belief, rendered
probable by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points; nor do I for a
moment pretend that any explanation could be offered of many such cases.
But after some preliminary remarks, I will discuss a few of the most
striking classes of facts; namely, the existence of the same species
on the summits of distant mountain-ranges, and at distant points in the
arctic and antarctic regions; and secondly (in the following chapter),
the wide distribution of freshwater productions; and thirdly, the
occurrence of the same terrestrial species on islands and on the
mainland, though separated by hundreds of miles of open sea. If the
existence of the same species at distant and isolated points of the
earth's surface, can in many instances be explained on the view of each
species having migrated from a single birthplace; then, considering our
ignorance with respect to former climatal and