Chap. VI. 
TRANSITIONAL VARIETIES. 
175 
it is quite remarkable how abruptly, as Alph. de Can¬ 
dolle has observed, a common alpine species disap¬ 
pears. The same fact has been noticed by E. 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 con¬ 
fines 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. 
If I am right in believing that allied or representative 
species, when inhabiting a continuous area, are gene¬ 
rally so distributed that each has a wide range, with 
a comparatively narrow neutral territory between them, 
in which they become rather suddenly rarer and rarer; 
then, as varieties do not essentially differ from species. 
