a_,
_b_, bones of fore-arm; 5, bones of the wrist; 6, bones of the hand;
7, bones of the fingers.]

[Illustration: 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.

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.
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.

[Illustration: 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.]

[Illustration: FIG. 79.--Paddle of a Whale.]

Such, then, is the most generalized as it is the most ancient type of
vertebrate limb above the class of fishes. Obviously it is a type
suited only to aquatic life. Consequen