182 Darwin, and after Darwin. 
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 de- 
scendants, the mammals. Moreover, when we examine 
the whole mammalian series, we find that the required 
modifications must have taken place in slightly differ- 
ent 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 not say anything more about the 
plantigrade division. But the digitigrade modification 
necessitated a change of structural plan, to the extent 
of raising the wrist and ankle joints off the ground, so 
as to make the quadruped walk on its fingers and toes. 
We meet with an interesting case of this transition 
in the existing hare, which while at rest supports 
itself on the whole hind foot after the manner of a 
plantigrade animal, but when running does so upon 
the ends of its toes, after the manner of a digitigrade 
animal. 
It is of importance for us to note that this transi- 
tion from the original plantigrade to the more recent 
digitigrade type, has been carried out on two slightly 
different plans in two different lines of mamma- 
lian descent. The hoofed mammals—which are all 
digitigrade—are sub-classified as artiodactyls and 
perissodactyls, i.e. even-toed and odd-toed. Now, 
