a separate line of evolution. Perhaps only a single species took to the trees; for it is quite possible for a single form, in a new and advantageous habitat, to vary in time into a great number of species.

Of all this we can know nothing: but of one thing we may feel assured, which is that the plantigrade foot is the only one that could have developed into a grasping organ; such a development being impossible to the digitigrade or the hoofed animals. One can readily see how the habit of walking on the sole might tend to a spreading of the toes, in order to obtain a wider and firmer footing. And it is equally easy to see how a free and wide motion in the great toe would aid in this result. The animal may have been at first light in weight and able to support itself on its unchanged foot, but as it increased in size and weight it would need a firmer grasp, and the final result of spread[45]ing its toes for this purpose may well have been the opposable great toe.

It must be borne in mind, in this consideration, that the apes differ from the other tree-dwellers in being destitute of claws. The squirrels, the opossums, and other arboreal animals have sharp claws, by whose aid they can easily cling to the surface of the bark-covered boughs. The nails of the apes are incapable of affording them this service, and it is not easy to perceive how a foot like theirs could become adapted to locomotion in the trees otherwise than by the gaining of mobile action and grasping power in the toes.

The existing habits of the ape tribe lead us to the conclusion that the ancestral animal may have soon begun to seek support from upper limbs. The plantigrade foot is one capable of readily curving into an organ of support, and in the case of the forefoot the toes would tend to spread and gain flexibility of motion, and the first toe to become opposable to the others and yi