These are strange relations on the view that each species was
independently created, but are intelligible if each existed first as a
variety.
As each species tends by its geometrical rate of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by as much as they become more diversified in
habits and structure, so as to be able to seize on many and widely
different places in the economy of nature, there will be a constant
tendency in natural selection to preserve the most divergent offspring of
any one species. Hence during a long-continued course of modification, the
slight differences characteristic of varieties of the same species, tend to
be augmented into the greater differences characteristic of the species of
the same genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and thus
species are rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups within each class tend to
give birth to new and dominant forms; so that each large group tends to
become still larger, and at the same time more divergent in character. But
as all groups cannot thus go on increasing in size, for the world would not
hold them, the more dominant groups beat the less dominant. This tendency
in the large groups to go on increasing in size and diverging in character,
together with the inevitable contingency of much extinction, explains the
arrangement of all the forms of life in groups subordinate to groups, all
within a few great classes, which has prevailed throughout all time.
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