Chap. VI.] OF TEANSITIONAL VAEIETIES. 209 



and I will here only state that I believe the answer 

 mainly lies in the record being incomparably less 

 perfect than is generally supposed. The crust of the 

 earth is a vast museum ; but the natural collections 

 have been imperfectly made, and only at long intervals 

 of time. 



But it may be urged that when several closely-allied 

 species inhabit the same territory, we surely ought to 

 find at the present time many transitional forms. Let 

 us take a simple case : in travelling from north to 

 south over a continent, we generally meet at successive 

 intervals with closely allied or representative species, 

 evidently filling nearly the same place in the natural 

 economy of the land. These representative species often 

 meet and interlock ; and as the one becomes rarer and 

 rarer, the other becomes more and more frequent, till 

 the one replaces the other. But 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 metropolis 

 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 

 transitional 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 embedded there in a fossil condition. But in 

 the intermediate region, having intermediate conditions 

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

 mediate varieties ? This difficulty for a long time quite 



