408 
GEOGPixiPHICAL DISTEIBUTIOK. 
Chap. XIL 
If the difficulties be not insuperable in admitting 
that in the long course of time the individuals of the 
same species, and likewise of allied species, have pro¬ 
ceeded from some one source; then I think all the grand 
leading facts of geographical distribution are explicable 
on the theory of migration (generally of the more do¬ 
minant forms of life), together with subsequent modifi¬ 
cation and the multiplication of new forms. We can 
thus understand the high importance of barriers, whether 
of land or water, which separate our several zoological 
and botanical provinces. We can thus understand the 
localisation of sub-genera, genera, and families ; and how 
it is that under different latitudes, for instance in South 
America, the inhabitants of the plains and mountains, of 
the forests, marshes, and deserts, are in so mysterious 
a manner linked together by affinity, and are likewise 
linked to the extinct beings which formerly inhabited the 
same continent. Bearing in mind that the mutual rela¬ 
tion of organism to organism is of the highest import¬ 
ance, we can see why two areas having nearly the same 
physical conditions should often be inhabited by very 
different forms of life; for according to the length of time 
which has elapsed since new inhabitants entered one 
region; according to the nature of the communication 
which allowed certain forms and not others to enter, either 
in greater or lesser numbers; according or not, as those 
which entered happened to come in more or less direct 
competition with each other and with the aborigines; 
and according as the immigrants were capable of vary¬ 
ing more or less rapidly, there would ensue in different 
regions, independently of their physical conditions, infi¬ 
nitely diversified conditions of life,—there would be an 
almost endless amount of organic action and reaction,— 
and we should find, as we do find, some groups of beings 
greatly, and some only slightly modified,—some deve- 
