Chap. VI. 
TEANSITIONAL VAKIETIES. 
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 
