gravitation puts on its structure; and in producing,
as well as in arresting, every one of its movements, it has to overcome
eight times the inertia. Meanwhile, the muscles and bones have severally
increased their contractile and resisting powers, in proportion to the
areas of their transverse sections; and hence are severally but four times
as strong as they were. Thus, while the creature has doubled in height, and
while its ability to overcome forces has quadrupled, the forces it has to
overcome have grown eight times as great. Hence, to raise its body through
a given space, its muscles have to be contracted with twice the intensity,
at a double cost of matter expended." Again, as to the cost at which
nutriment is distributed through the body, and effete matters removed from
it, "Each increment of growth being added at the periphery of an organism,
the force expended in the transfer of matter must increase in a rapid
progression--a progression more rapid than that of the mass."

There is yet another point. Vast as may have been the time during which the
process of evolution has continued, it is nevertheless not infinite. Yet,
as every kind, on the Darwinian hypothesis, varies slightly but
indefinitely in every organ and every part of every organ, how very
generally must favourable variations as to the length of the neck have {29}
been accompanied by some unfavourable variation in some other part,
neutralizing the action of the favourable one, the latter, moreover, only
taking effect during these periods of drought! How often must not
individuals, favoured by a slightly increased length of neck, have failed
to enjoy the elevated foliage which they had not strength or endurance to
attain; while other individuals, exceptionally robust, could struggle on
yet further till they arrived at vegetation within their reach.

However, allowing this example to pass, many other instances will be found
to present great difficulties.

Let us take the cases of mimicry amongst lepidoptera and other insects. Of
this subject Mr. Wallace has given a most interesting and complete
account,[24] showing in how many and strange instances this superficial
resemblance by one creature to some other quite distinct creature acts as a
safeguard to the first. One or two instances must here suffice. In South
America there is a family of butterflies, termed _Heliconidae_, which are
very conspicuously coloured and slow in flight, and yet the individuals