Chap. VI.] OF TEANSITIONAL VAEIETIES. 211 



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 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 

 manner that each has a wide range, with a compara- 

 tively narrow neutral territory between them, in which 

 they become rather suddenly rarer and rarer ; then, as 

 varieties do not essentially differ from species, the same 

 rule will probably apply to both; and if we take a 

 varying species inhabiting a very large area, we shall have 



