350 
GEOGRAPHICAL DISTRIBUTION. 
Chap. XI. 
the American continent and in the American seas. 
We see in these facts some deep organic bond, prevail¬ 
ing throughout space and time, over the same areas of 
land and water, and independent of their physical con¬ 
ditions. The naturalist must feel little curiosity, who 
is not led. to inquire what this bond is. 
This bond, on my theory, is simply inheritance, that 
cause which alone, as far as we positively know, pro¬ 
duces organisms quite like, or, as we see in the case of 
varieties, nearly like each other. The dissimilarity of 
the inhabitants of different regions may be attributed 
to modification through natural selection, and in a quite 
subordinate degree to the direct infiuence of different 
physical conditions. The degree of dissimilarity will de¬ 
pend on the migration of the more dominant forms of life 
from one region into another having been effected with 
more or less ease, at periods more or less remote;—on 
the nature and number of the former immigrants ;— 
and on their action and reaction, in their mutual 
struggles for life ;—the relation of organism to organism 
being, as I have already often remarked, the most im¬ 
portant of all relations. Thus the high importance of 
barriers comes into play by checking migration ; as 
does time for the slow process of modification through 
natural selection. Widely-ranging species, abounding 
in individuals, which have already triumphed over many 
competitors in their own widely-extended homes will 
have the best chance of seizing on new places, when they 
spread into new countries. In their new homes they 
will be exposed to new conditions, and will frequently 
undergo further modification and improvement; and 
thus they will become still further victorious, and will 
produce groups of modified descendants. On this prin¬ 
ciple of inheritance with modification, we can under¬ 
stand how it is that sections of genera, whole genera. 
