Chap. XI. 
SINGLE CENTEES OF CEEATION. 
353 
tralia or South America ? The conditions of life are 
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 identi¬ 
cally 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 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 sec¬ 
tions 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 oppo¬ 
site 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 natu¬ 
ralists, that the view of each species having been pro¬ 
duced in one area alone, and having subsequently mi¬ 
grated from that area as far as its powers of migration 
and subsistence under past and present conditions per¬ 
mitted, 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 cer¬ 
tainly occurred within recent geological times, must 
have interrupted or rendered discontinuous the for- 
