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the number of species of shells,
→that from the middle part of this same period the number
of mammals, has not greatly or at all increased. 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
area must have a limit, depending so largely as it does on physical
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
cases
be rapid, whereas the production of new species
always be
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 principle often explained, present within a given period few
variations; consequently, the process of giving birth to new specific forms
thus be retarded. When any species becomes very rare, close interbreeding will help
it;
authors have thought that this comes into play in accounting for the deterioration of
in Lithuania, of Red Deer in Scotland, and of
in Norway,
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. Alph. de Candolle has shown that those species which spread
tend generally to spread
very
widely;
→and, consequently, they will tend to
→supplant and exterminate
several species in several areas,
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