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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 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 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 {174} 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 it has been continuous during a long period.
Geology would lead us to believe that almost every continent has