DIFFICULTIES ON THEORY 157 



it has been continuous during- a long period. Geology 

 would lead us to believe that almost every continent 

 has been broken up into islands even during- the later 

 tertiary periods ; and in such islands distinct species 

 might have been separately formed without the possi- 

 bility of intermediate varieties existing in the inter- 

 mediate 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 denned 

 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 some- 

 what 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 dis- 

 appears. 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 condi- 

 tions of life as the all-important elements of distribu- 

 tion, 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 must see that the range of the inhabitants 



