160 ABSENCE OR RARITY 



if we compare these species where they intermingle, they 

 are generally as absolutely distinct from each other in every 

 detail of structure as are specimens taken from the metrop- 

 olis inhabited by each. By my theory these allied species 

 are descended from a common parent; and during the 

 process of modification, each has become adapted to the 

 conditions of life of its own region, and has supplanted and 

 exterminated its original parent-form and all the transi- 

 tional varieties between its past and present states. Hence 

 we ought not to expect at the present time to meet with 

 numerous transitional varieties in each region^ though they 

 must have existed there, and may be imbedded 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? 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 in- 

 ferring, 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 liave been separately formed 

 without the possibility of intermediate 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 continu- 

 ous 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 

 formerlv 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 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 territory proper 

 to each. We see the same fact in ascending mountains. 



