6. The number of palm-bones (metapodal) and toes deserves special notice. In fishes, and in some extinct swimming reptiles, these are or were very numerous, but in the earliest land-animals they became five. This is the number now in nearly all reptiles, and in all the more generalized mammals. It may be called the normal number for a walking animal. In very many mammals, such, for example, as the dog family, they are reduced to four, though the fifth often remains as a useless, rudimentary splint and dew-claw ([Fig. 6]), thus showing the process of dwindling in the ancestry. In hoofed animals the process of gradual diminution is shown even in existing forms, and still better in extinct forms. Confining ourselves, now, only to existing forms, in the elephant there are five palm-bones and toes, and in the hippopotamus there are four, all functional. In the hog ([Fig. 7]) there are still four, but two are behind the others and much smaller, and do not touch the ground—are not functional unless in soft ground. In the cow, deer, etc., the palm-bones are reduced to two, and these are consolidated into one (canon-bone), and the toes are reduced to two efficient and two useless rudiments. In the sheep and the goat ([Fig. 8]) these useless rudiments are dropped, and there are two only. Finally, in the horse ([Fig. 9]), the toes are reduced to one, although the palm-bones are still three, two of them, however, being reduced to rudimentary splints.
How is it with birds? Have these also palm-bones and fingers? Yes, in birds ([Fig. 11]) there are three palm-bones and three fingers (the fourth and fifth being wanting); one of them—the thumb—is free, and sometimes carries a claw. In the earliest known and most reptilian bird, the archæopteryx ([Fig. 12]), all the three fingers are free, have the full number of joints, and all of them carry claws. In the embryo of living birds the fingers are all free, as in the archæopteryx.
Fig. 19.—Restoration of Rhamphorhynchus phyllurus (after Marsh). One-seventh natural size.
7. Observe, finally, as an admirable illustration of different adaptative modifications for the same purpose—flight—the structure of the manus of flying animals. In the bat ([Fig. 10]), the flat flying-plane is made by enormous elongation of the palm-bones and finger-bones, their wide separation and the stretching of a thin membrane between them. In the pterosaurs, or extinct flying reptiles ([Fig. 13]), one finger only is greatly enlarged and elongated, and the flying-membrane is stretched between it and the hind-leg ([Fig. 19]), while the other three fingers are free and provided with claws. If it be asked which finger is it that is so greatly enlarged in this animal, we answer, it is the little finger. In birds, on the contrary, the manus is consolidated to the last degree, to form a strong basis for attachments for the quills which form the flying-plane, and which are themselves extreme modifications of the scales of reptiles. But throughout all these extreme modifications the same essential structure is detectable.
It is perhaps unnecessary to dwell upon the still greater modifications of limbs for swimming, as in the whale ([Fig. 16]), the ichthyosaur, mosasaur ([Fig. 18]), and the fish ([Fig. 17]). A careful inspection of the figures, after what we have said, will be sufficient to explain them. In the fish alone the upper segments of the limb, viz., shoulder-girdle and humerus, are wanting, not being yet introduced, and the manus is not yet differentiated into palm-bones and fingers, and the fingers are indefinitely multiplied. All these characters are indications of low position in the scale of evolution. The earliest vertebrates were fishes. Limbs were not yet completely formed. In embryos of higher animals, also, the outer segments are first formed.
Hind-Limbs.—[Figs. 20 to 24] represent, in a similar way, the hind-limbs of several animals—in this case all mammals. As before, corresponding parts are similarly lettered, and a dotted line is carried through certain prominent parts, especially the knee, heel, instep, and toes. By careful inspection the figures explain themselves. Nevertheless, it will be well to draw special attention to several of the more important points:
Figs. 20–24.—20. Hind-limb of man. 21. Monkey. 22. Dog. 23. Sheep. 24. Horse.