more of its inhabitants. And such new places will depend on slow
changes of climate, or on the occasional immigration of new
inhabitants, and, probably, in a still more important degree, on some
of the old inhabitants becoming slowly modified, with the new forms
thus produced and the old ones acting and reacting on each other. So
that, in any one region and at any one time, we ought only to see a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.
Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties between
the several representative species and their common parent, must
formerly have existed in each broken portion of the land, but these
links will have been supplanted and exterminated during the process of
natural selection, so that they will no longer exist in a living state.
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
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