DIFFICULTIES OF THE THEORY 237 



dredge. To those who look at climate and the physical 

 conditions of life as the all-important elements of distri- 

 bution, 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 

 see that the range of the inhabitants of any country by 

 no means exclusively depends on insensibly changing 

 physical conditions, but in a large part on the presence 

 of other species, on which it lives, or by which it is 

 destroyed, or with which it comes into competition; and 

 as these species are already defined objects, 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 nature of the 

 seasons, be extremely liable to utter extermination; and 

 thus its geographical range will come to be still more 

 sharply defined. 



As allied or representative species, when inhabiting a 

 continuous area, are generally distributed in such a man- 

 ner 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 essent' Jly differ from species, the same rule will 

 probably apply to both; and if we take a varying species 



