DIFFICULTIES OF THE THEORY 235 



be more convenient to discuss this question in the chapter 

 on the Imperfection of the Geological Recor d; and I will 

 here only state that I believe the answer mainly lies in 

 the record being incomparably less perfect than is gener- 

 ally 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 commoa pareat; 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 imbedded there in a fossil condition. But in 

 the intermediate region, having intermediate conditions 

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



