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nearly the same, so that a multitude of European animals and plants have become naturalised in America and Australia; and some of the aboriginal plants are identically the same at these distant points of 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
and broken
The great and striking influence
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
and have not been able to migrate to the
side. Some few families, many sub-families, very many genera, and a still greater number of sections of
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
→confined to one area. What a strange anomaly it would
→be, if, when coming
one step lower in the series,
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!
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| 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
rendered discontinuous the
continuous range of many species. So that we are reduced to consider whether the exceptions to
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