tly, when aquatic Vertebrata began
to become terrestrial, the type would have needed modification in order
to serve for terrestrial locomotion. In particular, it would have needed
to gain in consolidation and in firmness, which means that it would have
needed also to become jointed. Accordingly, we find that this archaic
type gave place in land-reptiles to the exigencies of these
requirements. Here for example is a diagram, copied from Gegenbaur, of
the right fore-foot of _Chelydra serpentina_ (Fig. 78). As compared with
the homologous limb of its purely aquatic predecessor, there is to be
noticed the disappearance of one of the six rows of small bones, a
confluence of some of the remainder in the other five rows, a
duplication of the arm-bone into a radius and ulna, in order to admit of
jointed rotation of the hand, and a general disposition of the small
bones below these arm-bones, which clearly foreshadows the joint of the
wrist. Indeed, in this fore-foot of _Chelydra_, a child could trace all
the principal homologies of the mammalian counterpart, growing, like the
next stage in a dissolving view, out of the primitive paddle of
_Baptanodon_--namely, first the radius and ulna, next the carpals, then
the meta-carpals, and, lastly, the three phalanges in each of the five
digits.

Such a type of foot no doubt admirably meets the requirements of slow
reptilian locomotion over swampy ground. But for anything like rapid
locomotion over hard and uneven ground, greater modifications would be
needed. Such modifications, however, need not be other in kind: it is
enough that they should continue in the same line of advance, so as to
reach a higher degree of firmness, combined with better joints.
Accordingly we find that this took place, not indeed among reptiles,
whose habits of cold-blooded life have not changed, but among their
warm-blooded descendants, the mammals. Moreover, when we examine the
whole mammalian series, we find that the required modifications must
have taken place in slightly different ways in three lines of descent
simultaneously. We have first the plantigrade and digitigrade
modifications already mentioned (pp. 178, 179) Of these the plantigrade
walking entailed least change, because most resembling the ancestral or
lizard-like mode of progression. All that was here needed was a general
improvement as to relative lengths of bones, with greater consolidation
and greater flexibility of joints. Therefore I need