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.