n animal having, for instance,
the structure and habits of a bat, could have been formed by the
modification of some other animal with widely different habits and
structure? Can we believe that natural selection could produce, on
the one hand, an organ of trifling importance, such as the tail of a
giraffe, which serves as a fly-flapper, and, on the other hand, an organ
so wonderful as the eye?

Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to the instinct which leads the bee to make
cells, and which has practically anticipated the discoveries of profound
mathematicians?

Fourthly, how can we account for species, when crossed, being sterile
and producing sterile offspring, whereas, when varieties are crossed,
their fertility is unimpaired?

The two first heads will be here discussed; some miscellaneous
objections in the following chapter; Instinct and Hybridism in the two
succeeding chapters.

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 fi