ese difficulties and objections may be classed under the following
heads:--Firstly, why, if species have descended from other species by
insensibly fine gradations, do we not everywhere see innumerable
transitional forms? Why is not all nature in confusion instead of the
species being, as we see them, well defined?

Secondly, is it possible that an animal having, for instance, the structure
and habits of a bat, could have been formed by the modification of some
animal with wholly different habits? Can we believe that natural selection
could produce, on the one hand, organs of trifling importance, such as the
tail of a giraffe, which serves as a fly-flapper, and, on the other hand,
organs of {172} such wonderful structure, as the eye, of which we hardly as
yet fully understand the inimitable perfection?

Thirdly, can instincts be acquired and modified through natural selection?
What shall we say to so marvellous an instinct as that which leads the bee
to make cells, 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 shall be here discussed--Instinct and Hybridism in
separate 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 or other less-favoured forms with
which it comes into competition. Thus 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 inhabi