156 ON THE ORIGIN OF SPECIES 



sediment is deposited on the shallow bed of the sea, 

 whilst it slowly subsides. These contingencies will 

 concur only rarely, and after enormously long intervals. 

 Whilst the bed of the sea is stationary or is rising, or 

 when very little sediment is being deposited, there will 

 be blanks in our geological history. The crust of the 

 earth is a vast museum ; but the natural collections 

 have been made only at intervals of time immensely 

 remote. 



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 succes- 

 sive 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 have descended from a common parent ; 

 and during the process of modification, each has be- 

 come adapted to the conditions of life of its own 

 region, and has supplanted and exterminated its 

 original parent and all the transitional varieties be- 

 tween 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 intermediate varieties ? 

 This difficulty 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 inferring, because an area is now continuous, that 



