of species, many European plants have become naturalised. But geology
shows us, that from an early part of the tertiary period the number of
species of shells, and 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 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. Alph. de Candolle has shown that those species which spread
widely tend generally to spread VERY widely, 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 southeast corner of Australia,
where, apparently, there are many invaders from different quarters of
the globe, the endemic Australian species have been greatly reduced in
number. How much weight to attribute to these several considerations I
will not pretend to say; but conjointly they must limit in each country
the tendency to an indefinite