In the third order, the climbers (Scansores), or, as they have been sometimes termed, the yoke-footed, to which the woodpecker and the parrot tribe belong, the structure of the foot is modified for the important purpose of enabling the creature to support itself in all positions upon the stems and branches of trees. In order to effect this the foot is made strong, short, and muscular, the claws sharp and curved, and the toes are placed in a very different position to that which they occupy in other orders, two of them being situated in front and two behind. By such a disposition, a strong, secure grasp is afforded; and thus it is that most birds belonging to this order are enabled to climb in a vertical position, and to effect movements like animals of the monkey tribe. In general these birds move very awkwardly upon a level surface, but in one or two species the structure of the foot is so modified that they can run upon the ground with great speed. This is the case with the travellers’ friend of South America, and with the ground parrot of New South Wales; and these creatures have accordingly slender toes so constructed, that one of the two placed behind can be brought in front at will, so that a larger and more secure basis of support is thus afforded.
In the fourth order the scratchers (Rasores), to which the common fowl, the pheasant, and the pigeon belong, the foot is adapted for an instrument of active locomotion, as well as for the purpose of procuring food by scratching in the earth. Hence we find it strong, with a long muscular leg, and four toes, three before and one behind, covered with broad scales and armed with short claws. The thumb, so necessary for perching and climbing, is in birds of this order short, rudimentary, and attached high up in the leg, so that the whole foot being placed flat upon the ground, affords a very firm support to the animal.
In the fifth order of birds, the runners (Cursores), to which the ostrich, the emu, and the cassowary belong, the structure of the foot is wholly that of a powerful instrument of locomotion. The leg is long, the thigh strong and muscular, and the foot short and compact. The toes which never exceed two or three in number, and of which the second is the largest, are placed all in front, and armed with very short claws, or, as in the African ostrich, with the outer toe destitute of a claw. The scales are limited to the upper surface of the foot, the under surface being flexible and occupied by an elastic pad upon which the member rests. Such a conformation of limb, which would be wholly useless for perching or climbing, is admirably fitted for running—witness the swiftness of the ostrich—a creature equalling and even surpassing the fleetest horse in speed.
In the sixth order, the waders (Grallatores), or stilt birds, as the stork, the crane, the heron, &c., the leg is long and slender, and the foot weak and covered with shield-shaped scales. The toes, four in number, are long and straight, three being placed anteriorly, and a shorter one (wanting in the plovers and lapwings) behind; they are frequently palmated at their base, as in the stork; or, as in the middle claw of the heron, toothed or notched like a saw; while sometimes they have a narrow-winged appendage, running along the side, as in the sandpipers. In search of their food, insects and small fish, the individuals of this order frequent marshy places and shallow waters, and many of them, as the stork and heron, are remarkable for the unerring certainty with which they strike and seize their small finny prey.
In the last order, the swimmers (Natatores), called also web-footed birds, to which the goose, duck, and gull belong, the foot is modified to assist in swimming. The legs are short; the four toes, all placed in front, are united by a broad membranous expansion, and armed with claws, the thumb being likewise brought in front, so that a greater extent of surface may be given to this oar-like member. A foot thus constructed would be wholly unsuited for perching, running, or grasping, but it is well adapted to the wants and habits of these aquatic birds.
Mammals.—It is but in animals who suckle their young (Mammalia) that the extremities attain their greatest state of developement, and it is in the higher orders alone that they become useful as organs of prehension, their office being limited in most of the lower tribes to support and locomotion. In the lowest order of the series, or whale tribe (Cetacea), the members, in the number of pieces of which they are composed, bear a considerable resemblance to the fins of fishes, though they are somewhat differently constructed, according as the animal may be destined exclusively for an aquatic existence, like the common sperm whale, or be more or less herbivorous, and consequently terrestrial in its habits, like the dugong. In the former case, the extremities are converted into broad paddles adapted for swimming, and the feet are webbed; in the latter, the members are better developed, the toes being free, and furnished with nails at their termination.
Of those orders of Mammalia which are provided with four members—the ordinary quadrupeds—some are furnished with a foot which appears to be entirely without toes, their place being supplied by a mass of horny tissue—the hoof. This is the case with the ruminants—animals that chew the cud—as the cow, and with the horse; but in the latter, as the name of the order implies (Solidungula), the hoof is solid, while in the ruminants it is cleft; and hence by some naturalists these latter animals have been termed Bisulca. Some slight differences are perceptible in the construction of the feet of ruminants: thus the chamois, the antelope, and other of the deer tribe, are furnished with an additional toe, which, projecting backwards, affords a firmer support to the body. In the thick-skinned animals (Pachydermata) the toes can hardly be said to be free, being still partially enclosed in the tough skin. It is from characters afforded by the foot that several tribes of animals belonging to this order are mainly characterised: thus the elephant has five toes to each foot, the rhinoceros three, and the hog and hippopotamus four. Among those of the mammalia which are characterised by the presence of four members with distinct and separate toes and nails, but few are furnished with a true hand—that is, a member with a thumb capable of being brought into apposition with the other fingers. Carnivorous animals have either a foot with sharp recurved and retractile claws, as the cat and tiger, or with straight blunt claws, not retractile, as the bear; or the feet are small, almost completely enveloped in the skin, and furnished with a membranous expansion connecting the toes, as in the aquatic seal and walrus, or river-horse: but in all these animals, although the fore-legs may be more or less employed as instruments of prehension, they are not used for the purpose of conveying food to the mouth. In some of the gnawing animals, as the squirrel, the anterior extremity commences to assume the function of a hand; and in a few of the kangaroo tribe, such as the opossum, the hinder extremities are provided with a thumb capable of being brought into apposition with the other toes. The hand of bats, which serves these animals not merely for flying, but also supplies the place of eyes and ears, enabling them to avoid objects during the obscurity of night, is furnished with a sense of touch extremely delicate. It is in man and the monkey tribe only that we meet with a true and perfect hand. In the latter, both the fore and hind extremities are constructed upon the same general principle as the hand of man; and hence these animals have been styled four-handed (Quadrumana).
“In man the upper members are disposed in a manner the most favourable for the exercise of their functions as organs of prehension and of touch: the fingers are long and flexible; they have all, with the occasional exception of the ring-finger, separate movements, which other animals (even those provided with hands) have not; the thumb, which is apposable to these latter, is longer in proportion to the fingers than in monkeys, and consequently can more readily apply itself to the extremity of their palm or surface, and better seize small objects. The nails, which are large and flat, cover the back part only of the extremity of the fingers, so as to furnish a support to the organs of touch without in the least depriving them of their delicacy. The entire hand can execute rotatory movements the most extended; and to the arm which supports it is afforded a solid attachment by means of a large shoulder or blade-bone and a long collar-bone.” The statement of Milne Edwards,[3] then, does not appear far removed from the truth, “That the faculties of the Mammalia are the more elevated in proportion as their members are the better constructed for prehension and for touch.”