236 THE ORIGIN OF SPECIES 



(Hate varieties? Tliis diHiculty for a long time quite 

 confounded me. But I think it can be in largo 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 lon^^periodL 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 f orm of th e land ar^^l 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 condi- 

 tion 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 comparison with the terri- 

 tory 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 



