What then checks an
indefinite increase in the number of species? The amount of life (I do not
mean the number of specific forms) supported on an area must have a limit,
depending so largely as it does on physical conditions; therefore, if an
area be inhabited by very many species, each or nearly each species will be
represented by few individuals; and such species will be liable to
extermination from accidental fluctuations in the nature of the seasons or
in the number of their enemies. The process of extermination in such cases
would be rapid, whereas the production of new species must always be slow.
Imagine the extreme case of as many species as individuals in England, and
the first severe winter or very dry summer would exterminate thousands on
thousands of species. Rare species, and each species will become rare if
the number of species in any country becomes indefinitely increased, will,
on the principal often explained, present within a given period few
favourable variations; consequently, the process of giving birth to new
specific forms would thus be retarded. When any species becomes very rare,
close interbreeding will help to exterminate it; authors have thought that
this comes into play in accounting for the deterioration of the aurochs in
Lithuania, of red deer in Scotland and of bears in Norway, etc. Lastly,
and this I am inclined to think is the most important element, a dominant
species, which has already beaten many competitors in its own home, will
tend to spread and supplant many others.
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