extinction and natural selection will, as we have seen, go hand in
hand. Hence, if we look at each species as descended from some other
unknown form, both the parent and all the transitional varieties will
generally have been exterminated by the very process of formation and
perfection of the new form.
But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers in the
crust of the earth? It will be much more convenient to discuss this
question in the chapter on the Imperfection of the geological record;
and I will here only state that I believe the answer mainly lies in the
record being incomparably less perfect than is generally supposed; the
imperfection of the record being chiefly due to organic beings not
inhabiting
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
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