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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
→become in any country
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, and thus check the inordinate increase of specific forms throughout the world. Dr. Hooker has recently shown that in the
corner of Australia, where, apparently, there are many invaders from different quarters of the
the endemic Australian species have been greatly reduced in number. How much weight to attribute to these several considerations
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