126 ABSENCE OR RAEITY [CHAP. VL 



This difficulty for a long time quite confounded me. But I think 

 it can be in large part explained. 



In the first place we should be extremely cautious in inferring, 

 because an area is now continuous, that it has been continuous 

 during a long period. Geology would lead us to believe that most 

 continents have been broken up into islands even during the later 

 tertiary periods ; and in such islands distinct species might have 

 been separately formed without the possibility of intermediate 

 varieties existing in the intermediate zones. By changes in the 

 form of the land and of climate, marine areas now continuous must 

 often have existed within recent times in a far less continuous and 

 uniform condition than at present. But I will pass over this way 

 of escaping from the difficulty ; for I believe that many perfectly 

 defined species have been formed on strictly continuous areas; 

 though I do not doubt that the formerly broken condition of areas 

 now continuous, has played an important part in the formation of 

 new species, more especially with freely-crossing and wandering 

 animals. 



In looking at species as they are now distributed over a wide 

 area, we generally find them tolerably numerous over a large 

 territory, then becoming somewhat abruptly rarer and rarer on 

 the confines, and finally disappearing. Hence the neutral territory 

 between two representative species is generally narrow in com- 

 parison with the territory proper to each. We see the same fact 

 in ascending mountains, and sometimes it is quite remarkable how 

 abruptly, as Alph. de Candolle has observed, a common alpine 

 species disappears. 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 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, 



