138 THE ORIGIN OF SPECIES 



now find closely linking intermediate varieties? 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 va- 

 rieties 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 uni- 

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

 escaping from the difficulty; for I believe that many perfectly de- 

 fined 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 terri- 

 tory, then becoming somewhat abruptly rarer and rarer on the 

 confines, and finally disappearing. Hence the neutral territory be- 

 tween two representative species is generally narrow in comparison 

 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 disap- 

 pears. 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 in- 

 crease 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 com- 

 petition; and as these species are already defined objects, not 

 blending one into another by insensible gradations, the range of 



