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In consequence of the proportions and mobility of the thumb, it is what is termed "opposable"; in other words, its extremity can, with the greatest ease, be brought into contact with the extremities of any of the fingers; a property upon which the possibility of our carrying into effect the conceptions of the mind so largely depends.
"But you, who have opened the books and who share my awful confidence you know him for what he is, brother to you and the dust, a cosmic joke, a sport of chemistry, a garmented beast that arose out of the ruck of screaming beastliness by virtue and accident of two opposable great toes. He is brother as well to the gorilla and the chimpanzee.
These early drifts we conjecture and know must have occurred, just as we know that the first upright-walking brutes were descended from some kin of the quadrumana through having developed "a pair of great toes out of two opposable thumbs."
And as to the foot, the great toe of the Marmoset is still more insignificant in proportion than that of the Orang while in the Lemurs it is very large, and as completely thumb-like and opposable as in the Gorilla but in these animals the second toe is often irregularly modified, and in some species the two principal bones of the tarsus, the 'astragalus' and the 'os calcis', are so immensely elongated as to render the foot, so far, totally unlike that of any other mammal.
Even now we have one living form, the curious Galeopithecus or flying lemur, which has only recently been separated from the lemurs, with which it was formerly united, to be classed as one of the insectivora; and it is only among the Opossums and some other marsupials that we again find hand-like feet with opposable thumbs, which are such a curious and constant feature of the monkey tribe.
These have become accustomed to an upright gait in their climbing; hence the feet are used for supporting the body and the hands for grasping. Both thumb and great toe are opposable; but the foot is a true foot, and the hand a true hand, in anatomical structure. The face, hands, and feet have mainly lost the covering of hair.
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 yield a more complete grasping power.
These are rather small-sized animals, with round heads and with moderately long tails. They are very active and intelligent, their limbs are not so long as in the preceding group, and though they have five fingers on each hand and foot, the hands have weak and hardly opposable thumbs.
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 spreading its toes for this purpose may well have been the opposable great toe.
These give place to a membrane like that which supersedes gills in the development of birds and reptiles; the heart is at first a simple pulsating chamber like that in worms; the backbone is prolonged into a movable tail; the great toe is extended, or opposable, like our thumbs, and like the toes of apes; the body three months before birth is covered all over with hair except on the palms and soles.
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