Therefore during the
modification of the descendants of any one species, and during the
incessant struggle of all species to increase in numbers, the more
diversified these descendants become, the better will be their chance
of succeeding in the battle of life. Thus the small differences
distinguishing varieties of the same species, will steadily tend to
increase till they come to equal the greater differences between
species of the same genus, or even of distinct genera.
We have seen that it is the common, the widely-diffused, and
widely-ranging species, belonging to the larger genera, which vary
most; and these will tend to transmit to their modified offspring that
superiority which now makes them dominant in their own countries.
Natural selection, as has just been remarked, leads to divergence of
character and to much extinction of the less improved and intermediate
forms of life. On these principles, I believe, the nature of the
affinities of all organic beings may be explained. It is a truly
wonderful fact--the wonder of which we are apt to overlook from
familiarity--that all animals and all plants throughout all time and
space should be related to each other in group subordinate to group,
in the manner which we everywhere behold--namely, varieties of the
same species most closely related together, species of the same genus
less closely and unequally related together, forming sections and
sub-genera, species of distinct genera much less closely related, and
genera related in different degrees, forming sub-families, families,
orders, sub-classes, and classes.
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