lst 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 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
been broken up into islands even during the later tertiary periods;
and in such islands distinct species might have been separately formed
without the possibility of intermediate varieties existing in the
intermediate zones. By changes in the form of the land and of climate,
marine areas now continuous must often have existed within recent time