the northern and southern hemispheres? The answer, as I believe, is,
that mammals have not been able to migrate, whereas some plants, from
their varied means of dispersal, have migrated across the vast and
broken interspace. 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. 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
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