210 ABSENCE OR RARITY [Chap. VL 



founded me. But I think it can be in large part ex- 

 plained. 



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 inter- 

 mediate 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 wander- 

 ing 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 ter- 

 ritory proper to each. We see the same fact in ascend- 

 ing mountains, and sometimes it is quite remarkable 

 how abruptly, as Alph. de CandoUe 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 



