Fig. 76.—Anterior limb of Man, Dog, Hog, Sheep, and Horse. (After Le Conte.) Sc, shoulder-blade; c, coracoid; a, b, bones of fore-arm; 5, bones of the wrist; 6, bones of the hand; 7, bones of the fingers.
Fig. 77.—Posterior limb of Man, Monkey, Dog, Sheep and Horse. (After Le Conte.) 1, Hip-joint; 2, thigh-bone; 3, knee-joint; 4, bones of leg; 5, ankle-joint; 6, bones of foot; 7, bones of toes.
I will now proceed to detail the history of mammalian limbs, as this has been recorded for us in fossil remains.
Fig. 78.—A, posterior limb of Baptanodon discus. (After Marsh.) F, thigh-bone; I to VI, undifferentiated bones of the leg and foot. B, anterior limb of Chelydra serpentina. (After Gegenbaur.) U and R, bones of the fore-arm; I to V, fully differentiated bones of the hand, following those of the wrist.
The most generalized or primitive types of limb hitherto discovered in any vertebrated animal above the class of fishes, are those which are met with in some of the extinct aquatic reptiles. Here, for instance, is a diagram of the left hind limb of Baptanodon discus (Fig. 78). It has six rows of little symmetrical bones springing from a leg-like origin.
Fig. 79.—Paddle of a Whale. But the whole structure resembles the fin of a fish about as nearly as it does the leg of a mammal. For not only are there six rows of bones, instead of five, suggestive of the numerous rays which characterise the fin of a fish; but the structure as a whole, having been covered over with blubber and skin, was throughout flexible and unjointed—thus in function, even more than in structure, resembling a fin. In this respect, also, it must have resembled the paddle of a whale (see Fig. 79); but of course the great difference will be noted, that the paddle of a whale reveals the dwindled though still clearly typical bones of a true mammalian limb; so that although in outward form and function these two paddles are alike, their inward structure clearly shows that while the one testifies to the absence of evolution, the other testifies to the presence of degeneration. If the paddle of Baptanodon had occurred in a whale, or the paddle of a whale had occurred in Baptanodon, either fact would in itself have been well-nigh destructive of the whole theory of evolution.