The objection which Cuvier raised to attempting the establishment of strict homologies between the bones of the thoracic and pelvic extremities, namely, "that the articulation of the leg upon the thigh is very different from that of the forearm upon the arm, because the movements have become so different," he himself answers in the following passage : after tracing the varieties in form in the bones of the forearm through the mammalian series, the position of the radius in front of the ulna in the pachyderms (rhinoceros, tapir, &c), the coalescence of the ulna to the radius, and the obliteration of the carpal extremity in the ruminants, the fixed prone position of the phalanges in the ruminants and solipedes, he adds, "We see from this series of conformations that the rotation of the hand becomes the more difficult, the less the animal uses it for prehension, and the more exclusively it employs its anterior extremity for support (station) and progression. In fact these last usages exact a constant pronation, and a firmness incompatible with the possibility of supination."* He might have added, they bring the anterior extremities to an arrangement of their component parts so similar to that of the posterior extremities, that their points of resemblance can be traced in every respect, but it must be remembered that the flexures of the knee and elbow are reversed ; that which is flexion in the one is extension in the other, and vice versa; but upon this point further remarks will be made after the consideration of the bones composing the knee-joint.
* Lemons d'Anatomie comparee, vol. i., p. 408.