1 82 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, L e. even-toed and odd-toed. Now, 



