CHAPTER VI. DIFFICULTIES OF THE THEORY.
2. ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to take
the place of, and finally to exterminate, its own less improved parent-form
and other less-favoured forms with which it comes into competition. Thus
extinction and natural selection go hand in hand. Hence, if we look at
each species as descended from some unknown form, both the parent and all
the transitional varieties will generally have been exterminated by the
very process of the 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 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 crust of the earth is a vast
museum; but the natural collections have been imperfectly made, and only at
long intervals of time.
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 are 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-form
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.