Thirdly, when two or more varieties have been formed in different
portions of a strictly continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these
intermediate varieties will, from reasons already assigned (namely
from what we know of the actual distribution of closely allied or
representative species, and likewise of acknowledged varieties), exist
in the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate varieties
will be liable to accidental extermination; and during the process of
further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they connect;
for these from existing in greater numbers will, in the aggregate,
present more variation, and thus be further improved through natural
selection and gain further advantages.
Lastly, looking not to any one time, but to all time, if my theory be
true, numberless intermediate varieties, linking most closely all the
species of the same group together, must assuredly have existed; but
the very process of natural selection constantly tends, as has been so
often remarked, to exterminate the parent forms and the intermediate
links. Consequently evidence of their former existence could be found
only amongst fossil remains, which are preserved, as we shall in a
future chapter attempt to show, in an extremely imperfect and
intermittent record.
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