|
it is quite remarkable how abruptly, as Alph.
Candolle has observed, a common alpine species disappears. The same fact has been noticed by
in sounding the depths of the sea with the dredge. To those who look at climate and the physical conditions of life as the all-important elements of distribution, these facts ought to cause surprise, as climate and height or depth graduate away insensibly. But when we bear in mind that almost every species, even in its metropolis, would increase immensely in numbers, were it not for other competing species; that nearly all either prey on or serve as prey for others; in short, that each organic being is either directly or indirectly related in the most important manner to other organic
we
see that the range of the inhabitants of any country by no means exclusively depends on insensibly changing physical conditions, but in
part on the presence of other species, on which it
or by which it is destroyed, or with which it comes into competition; and as these species are already defined
→(however they may have become so),
not blending one into another by insensible gradations, the range of any one species, depending as it does on the range of others, will tend to be sharply defined. Moreover, each species on the confines of its range, where it exists in lessened numbers,
during fluctuations in the number of its enemies or of its prey, or in the
→seasons,
be extremely liable to utter extermination; and thus its geographical range will come to be still more sharply defined. |
