Coupled with the increasing perfection of the foot as an organ used merely for the support of the body, certain interesting changes have taken place in the arrangement with regard to each other of the several bonelets of the wrist and ankle. It has been held by Cope and others that the truly primitive disposition of these bones was that presented to us by certain early types, such as _Meniscotherium_ or the existing elephant or _Hyrax_. In these animals there is (see Fig. 112) a serial {197} arrangement of these bones, the distal bones only, or very nearly only, articulating with the corresponding bones in the upper series. In the modern types (cf. Fig. 113) there is, on the other hand, an interlocking, so that the bones of the distal series articulate with two of those of the proximal series. By this is produced, as it would appear, a much firmer foot, less liable to "give" under pressure, and thus more fitted for an animal that runs. It is the same principle as that adopted in the laying of bricks. The actual stress and strain of impact has been held responsible for those changes. An equally ingenious and possibly truer explanation of the undoubted facts has lately been advanced by Mr. W. D. {198} Matthew.[121] He has pointed out that in some ancient Ungulates the carpus is not serial but interlocking, even in forms which belong to the earliest Eocene groups, such as the genus _Protolambda_ among the Amblypoda. Now in the fore-foot of _Meniscotherium_ and the living _Hyrax_ there is a separate centrale which is wanting in the greater number of Ungulates. The absorption, that is the practical dropping out of this bone, would restore to an interlocking carpus the serial arrangement; while on the other hand, by the fusion of this bone with the scaphoid, the interlocking disposition would be maintained. [Illustration] FIG. 112.--Bones of the manus A, of the Indian Elephant, _Elephas indicus_. x 1/8. B, of the Cape Hyrax, _Hyrax capensis_. x 1. _c_, Cuneiform; _cc_, centrale; _l_, lunar; _m_, magnum; _p_, pisiform; _R_, radius; _td_, trapezoid; _tm_, trapezium; _s_, scaphoid; _u_, unciform; _U_, ulna. (From Flower's _Osteology_.) [Illustration] FIG. 113.--Bones of the manus A, of Rhinoceros, _Rhinoceros sumatrensis_. x 1/5. B, of Pig, _Sus scrofa_. x 1/3. Letters as in Fig. 112. (From Flower's _Osteology_.)