Chap. VI. TRANSITIONAL VARIETIES. 175 



it is quite remarkable how abruptly, as Alpli. De Can- 

 dolle has observed, a common alpine species disap- 

 pears. 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 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, 



