Chap. VI. TRANSITION AL VARIETIES. 173 



profound depths of the sea, and to their remains being 

 embedded and preserved to a future age only in masses 

 of sediment sufficiently thick and extensive to withstand 

 an enormous amount of future degradation ; and such 

 fossiliferous masses can be accumulated only where much 

 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 successive inter- 

 vals with closely allied or representative species, evi- 

 dently 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 fre- 

 quent, till the one replaces the other. But if we com- 

 pare 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 become 

 adapted to the conditions of life of its own region, and 

 has supplanted and exterminated its original parent 

 and all the transitional varieties between its past and 

 present states. Hence we ought not to expect at the 



