Ohap. VI.] OF TRANSITIONAL VARIETIES. 211 



the physical conditions of life as the all-important ele- 

 ments of distribution, these facts ought to cause surprise, 

 as climate and height or depth graduate away insen- 

 sibly. 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 in- 

 directly related in the most important manner to other 

 organic beings, — we see that the range of the inhabi- 

 tants 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 de- 

 fined objects, not blending one into another by insen- 

 sible 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, dur- 

 ing 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 geo- 

 graphical range will come to be still more sharply de- 

 fined. 



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 vary- 

 ing species inhabiting a very large area, we shall have 



