Jhap. VL] OP TRANSITIONAL VARIETIES. 209 



and I will here only state that I believe the answer 

 mainly lies in the record being incomparably less per- 

 fect 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 inter- 

 vals with closely allied or representative species, evi- 

 dently filling nearly the same place in the natural econ- 

 omy 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 em- 

 bedded there in a fossil condition. But in the inter- 

 mediate region, having intermediate conditions of life, 

 why do we not now find closely-linking intermediate 

 varieties? This difficulty for a long time quite con- 



