174 DIFFICULTIES ON THEORY. Chap. VI. 



present time to meet with numerous transitional vari- 

 eties in each region, though they must have existed 

 there, and may be embedded there in a fossil condition. 

 But in the intermediate region, having intermediate 

 conditions of life, why do we not now find closely-linking 

 intermediate varieties ? Tins difficulty for a long time 

 quite confoimded 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 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 interme- 

 diate 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 forma- 

 tion 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 



