It is evident that, in what is called the evolution of animal forms, the foot came in suddenly when the backboned creatures began to live on the dry land—that is, with the frogs. How it came in is a question which still puzzles the phylogenists, who cannot find a sure pedigree for the frog. There it is, anyhow, and the remarkable point about it is that the foot of a frog is not a rudimentary thing, but an authentic standard foot, like the yard measure kept in the Tower of London, of which all other feet are copies or adaptations. This instrument, as part of the original outfit given to the pioneers of the brainy, backboned, and four-limbed races, when they were sent out to multiply and replenish the earth, is surely worth considering well. It consists essentially of a sole, or palm, made up of small bones and of five separate digits, each with several joints.
In the hind foot of a frog the toes are very long and webbed from point to point. In this it differs a good deal from the toad, and there is significance in the difference. The "heavy-gaited toad," satisfied with sour ants, hard beetles, and such other fare as it can easily pick up, and grown nasty in consequence, so that nothing seeks to eat it, has hobbled through life, like a plethoric old gentleman, until the present day, on its original feet. The more versatile and nimble-witted frog, seeking better diet and greater security of life, went back to the element in which it was bred, and, swimming much, became better fitted for swimming. The soft elastic skin between the fingers or toes is just the sort of tissue which responds most readily to inward impulses, and we find that the very same change has come about in those birds and beasts which live much in water. I know that this is not the accepted theory of evolution, but I am waiting till it shall become so. We all develop in the direction of our tendencies, and shall, I doubt not, be wise enough some day to give animals leave to do the same.
It seems strange that any creature, furnished with such tricky and adaptable instruments to go about the world with, should tire of them and wish to get rid of them, but so it happened at a very early stage. It must have been a consequence, I think, of growing too fast. Mark Twain remarked about a dachshund that it seemed to want another pair of legs in the middle to prevent it sagging. Now, some lizards are so long that they cannot keep from sagging, and their progress becomes a painful wriggle. But if you must go by wriggling, then what is the use of legs to knock against stems and stones? So some lizards have discarded two of their legs and some all four. Zoologically they are not snakes, but snakes are only a further advance in the same direction. That snakes did not start fair without legs is clear, for the python has to this day two tell-tale leg-bones buried in its flesh.
When we pass from reptiles to birds, lo! an astounding thing has happened. That there were flying reptiles in the fossil ages we know, and there are flying beasts in our own. But the wings of these are simple mechanical alterations, which the imagination of a child, or a savage, could explain.
The hands of a bat are hands still, and, though the fingers are hampered by their awkward gloves, the thumbs are free. The giant fruit bats of the tropics clamber about the trees quite acrobatically with their thumbs and feet.
That Apollyonic monster of the prime, the pterodactyl, did even better. Stretching on each little finger a lateen sail that would have served to waft a skiff across the Thames, it kept the rest of its hands for other uses. But what bearing has all this on the case of birds? Here is a whole sub-kingdom, as they call it, of the animal world which has unreservedly and irrevocably bartered one pair of its limbs for a flying-machine. The apparatus is made of feathers—a new invention, unknown to amphibian or saurian, whence obtained nobody can say—and these are grafted into the transformed frame of the old limbs. The bargain was worth making, for the winged bird at once soared away in all senses from the creeping things of earth, and became a more ethereal being; "like a blown flame, it rests upon the air, subdues it, surpasses it, outraces it; it is the air, conscious of itself, conquering itself, ruling itself." But the price was heavy. The bird must get through life with one pair of feet and its mouth. But this was all the bodily furniture of Charles François Felu, who, without arms, became a famous artist.
A friend of mine, standing behind him in a salon and watching him at work, saw him lay down his brush and, raising his foot to his head, take off his hat and scratch his crown with his great toe. My friend was nearly hypnotised by the sight, yet it scarcely strikes us as a wonder when a parrot, standing on one foot, takes its meals with the other. It is a wonder, and stamps the parrot as a bird of talent. A mine of hidden possibilities is in us all, but those who dig resolutely into it and bring out treasure are few.
And let us note that the art of standing began with birds. Frogs sit, and, as far as I know, every reptile, be it lizard, crocodile, alligator, or tortoise, lays its body on the ground when not actually carrying it. And these have each four fat legs. Contrast the flamingo, which, having only two, and those like willow wands, tucks up one of them and sleeps poised high on the other, like a tulip on its stem.
Note also that one toe has been altogether discarded by birds as superfluous. The germ, or bud, must be there, for the Dorking fowl has produced a fifth toe under some influence of the poultry-yard, but no natural bird has more than four. Except in swifts, which never perch, but cling to rocks and walls, one is turned backwards, and, by a cunning contrivance, the act of bending the leg draws them all automatically together. So a hen closes its toes at every step it takes, as if it grasped something, and, of course, when it settles down on its roost, they grasp that tight and hold it fast till morning. But to birds that do not perch this mechanism is only an encumbrance, so many of them, like the plovers, abolish the hind toe entirely, and the prince of all two-legged runners, the ostrich, has got rid of one of the front toes also, retaining only two.