(2.) It may be also objected, that the power of reaching upwards, acquired by the lengthening of the neck and legs, must have necessitated a considerable increase in the entire size and mass of the body (larger bones requiring stronger and more voluminous muscles and tendons, and these again necessitating larger nerves, more capacious blood-vessels, &c.), and it is very problematical whether the disadvantages thence arising would not, in times of scarcity, more than counterbalance the advantages.
For a considerable increase in the supply of food would be requisite on account of this increase in size and mass, while at the same time there would be a certain decrease in strength;
for, as Mr. Herbert Spencer says,[[23]] "It is demonstrable that the excess of absorbed over expended nutriment must, other things equal, become less as the size of an animal becomes greater. In similarly-shaped bodies, the masses vary as the cubes of the dimensions; whereas the strengths vary as the squares of the dimensions.".... "Supposing a creature which a year ago was one foot high, has now become two feet high, while it is unchanged in proportions and structure—what are the necessary concomitant changes that have taken place in it? It is eight times as heavy; that is to say, it has to resist eight times the strain which 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 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 Heliconidæ, which are very conspicuously coloured and slow in flight, and yet the individuals abound in prodigious numbers, and take no precautions to conceal themselves, even when at rest, during the night. Mr. Bates (the author of the very interesting work "The Naturalist on the River Amazons," and the discoverer of "Mimicry") found that these conspicuous butterflies had a very strong and disagreeable odour; so much so that any one handling them and squeezing them, as a collector must do, has his fingers stained and so infected by the smell, as to require time and much trouble to remove it.
It is suggested that this unpleasant quality is the cause of the abundance of the Heliconidæ; Mr. Bates and other observers reporting that they have never seen them attacked by the birds, reptiles, or insects which prey upon other lepidoptera.
Now it is a curious fact that very different South American
butterflies put on, as it were, the exact dress of these offensive beauties and mimic them even in their mode of flight.