180 ORIGIN OF SPECIES 



solutely distinct from each other in every detail of structure 

 as are specimens taken from the metropolis inhabited by each. 

 By my theory these allied species are descended from a com- 

 mon 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 transitional varieties between its past and 

 present states. Hence we ought not to expect at the present 



f"~-i:ime to meet with numerous transitional varieties in each re- 

 gion, though they must have existed there, and may be em- 

 bedded there in a fossil condition. But in the intermediate 

 region, having intermediate conditions of life, why do we not 

 how find closely-linking intermediate varieties? This diffi- 

 culty 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 have been separately formed without 

 the possibility 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 ex- 

 isted 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 per- 

 fectly defined species have been formed on strictly continu- 

 ous 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 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 gen- 

 erally 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 



