njecture what changes of
structure would be favourable to its increase in some new country. We
can, however, see in a general manner that various causes might have
interfered with the development of a long neck or proboscis. To reach
the foliage at a considerable height (without climbing, for which hoofed
animals are singularly ill-constructed) implies greatly increased
bulk of body; and we know that some areas support singularly few large
quadrupeds, for instance South America, though it is so luxuriant,
while South Africa abounds with them to an unparalleled degree. Why this
should be so we do not know; nor why the later tertiary periods should
have been much more favourable for their existence than the present
time. Whatever the causes may have been, we can see that certain
districts and times would have been much more favourable than others for
the development of so large a quadruped as the giraffe.

In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable that several other parts
should be modified and coadapted. Although every part of the body varies
slightly, it does not follow that the necessary parts should always
vary in the right direction and to the right degree. With the different
species of our domesticated animals we know that the parts vary in
a different manner and degree, and that some species are much more
variable than others. Even if the fitting variations did arise, it
does not follow that natural selection would be able to act on them and
produce a structure which apparently would be beneficial to the species.
For instance, if the number of individuals existing in a country is
determined chiefly through destruction by beasts of prey--by external
or internal parasites, etc.--as seems often to be the case, then natural
selection will be able to do little, or will be greatly retarded, in
modifying any particular structure for obtaining food. Lastly, natural
selection is a slow process, and the same favourable conditions must
long endure in order that any marked effect should thus be produced.
Except by assigning such general and vague reasons, we cannot explain
why, in many quarters of the world, hoofed quadrupeds have not acquired
much elongated necks or other means for browsing on the higher branches
of trees.

Objections of the same nature as the foregoing have been advanced by
many writers. In each case various causes, besides the general