Page - 180 - in The Origin of Species

180 ORIGIN OF SPECIES solutely distinct from each other in every detail of structure as are specimens taken from the metropolis inhabited by each. Bymy 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