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We have to consider, then, which are the
species that would be most likely to be so modified, while others, not
becoming modified, would succumb to the changed conditions and become
extinct.
The most important condition of all is, undoubtedly, that variations
should occur of sufficient amount, of a sufficiently diverse character,
and in a large number of individuals, so as to afford ample materials
for natural selection to act upon; and this, we have seen, does occur in
most, if not in all, large, wide-ranging, and dominant species. From
some of these, therefore, the new species adapted to the changed
conditions would usually be derived; and this would especially be the
case when the change of conditions was rather rapid, and when a
correspondingly rapid modification could alone save some species from
extinction. But when the change was very gradual, then even less
abundant and less widely distributed species might become modified into
new forms, more especially if the extinction of many of the rarer
species left vacant places in the economy of nature.


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