De
Candolle has observed, a common alpine species disappears. The same
fact has been noticed by Forbes 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 beings, we must see that the range of the inhabitants of any
country by no means exclusively depends on insensibly changing
physical conditions, but in large part on the presence of other
species, on which it depends, or by which it is destroyed, or with
which it comes into competition; and as these species are already
defined objects (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, will, 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.
Pages:
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220