But should any members of it be obliged to migrate, from scarcity of food or any other reason, to some new locality where somewhat different conditions prevail, structural alterations would soon begin to appear to suit those new conditions. Thus the giraffe, if we could trace its ancestry back, would probably be found to have originated in some animal not differing from the vast majority of quadrupeds in the relative proportions of its fore and hind quarters. But some members of this original species—or the whole species, owing to some change in their surroundings—found themselves obliged to rely largely for food on leaves growing at a considerable height. They stretched up to reach them, and a prolongation of the bones of the neck (the giraffe has only the usual seven cervical vertebræ) and of the fore-legs would ensue, especially in the young; this prolongation would be handed on by inheritance, and so by degrees the new type of animal would be evolved. The horn of the rhinoceros, the antlers of the stag, the canine teeth of beasts of prey, the flat grinders of ruminants, the flippers of the whale, the proboscis of the honey-feeding butterfly, the jaws of the ant or the beetle, and a host of other adaptations which seem obviously to owe their origin to the exercise of their functions, occur to the mind in confirmation of this theory. Besides Adaptation, we have what appears the strikingly confirmatory case of what is called Co-adaptation, where the variation of one organ or structure in an animal puts a strain upon other parts, which accordingly respond by auxiliary adaptations. Such co-adaptations are numerous in every animal structure, and, as we cannot suppose them to have all originated simultaneously and by chance, the conclusion drawn by Lamarckians is that one was produced by use, and, in the course of its development, produced the others in the same way. A typical case is that of the Irish elk. The enormous antlers of this beast, sometimes weighing a hundredweight, must have needed (besides other structural changes) a cervical ligament of immense size and power to support them, and from the peculiar structure of the cervical vertebræ it is demonstrable that such a ligament must have existed. What more natural than to suppose that the antlers were developed by fighting wild beasts of prey, combats between male elks, etc., and that then in their gradual growth, as the species was evolved, the ligament and the bony structure associated with it responded to the increasing strain. That is exactly what would happen in an individual. We have only to assume the heritability of modifications acquired by use to understand how these co-adaptations became constant characters in a species.