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: [Illustration: FIGS. 20-24.--20. Hind-limb of man. 21. Monkey. 22. Dog. 23. Sheep. 24. Horse.] 1. See, then, the position of the knee. The thigh-bone in man, monkeys, bears, and several other families of mammals, and all reptiles, is free from the body, and the knee is far removed and half-way down the limb (Figs. 20, 21). This is undoubtedly the original and normal condition of land-animals. But in all the more highly specialized and swifter animals the knee is brought nearer and nearer to the body, until, in the swiftest of all, such as the ruminants and the horse (Figs. 23, 24), it is high up on the side of the body, in the middle of what is usually called the thigh but which really includes the thigh and the upper part of the lower leg or shank.