D.— ZOOLOGY. 91 



have not changed. Dr. Matthew has produced evidence to show that in 

 Merychippus, the Mciocene genus of horse, tooth change took place at a 

 younger age than it does in modern horses ; the implication being that the 

 potential longevity was less than it now is. 



Thus the fact that Equus has proportionately some four times as much 

 tooth as Eohippus may mean no more than that it lives longer, and its 

 marvellous dentition may not be adaptive in the sense that it is specially 

 modified for the trituration of a new type of food. It may represent no 

 more than a reaction to the requirements of a large animal. 



Thus in its dentition the horse may show not a definite adaptation to 

 a special diet, but such an improvement as enables a large animal to live 

 longer than its small ancestor. I believe that most adaptations whose 

 history can be traced in fossil material are of a similar kind. The changes 

 which go on in the limbs of a horse do unquestionably result in the forma- 

 tion of a machine which is more efficient than that of the Eocene animal. 

 In this case each leg is designed for rapid motion, the single toe is better 

 fitted to stand the great stresses it receives than the three or four of 

 earlier form, the interlocking of the third metapodial with three distal 

 carpals or tarsals is clearly mechanically sounder than the old one to one 

 relationship, and the reduction of the moment of inertia of the limb 

 which results from the concentration of its muscles in the proximal half 

 is a considerable improvement. How far these adaptations have resulted 

 in an increased speed of galloping and how far they were necessary to 

 enable an animal of much greater bulk to maintain the same speed we 

 do not know, nor, unless a far more precise analysis of the whole 

 mechanism prove possible, shall we ever know. 



Whether a change which enables a mammal to become larger and to 

 have a greater potential longevity is an adaptation may be disputed. 

 Certainly it is very different from the usual conception of a structural 

 change fitting an animal for a definite type of life under particular circum- 

 stances. 



A large herbivorous animal of no higher speed than a small one suffers 

 from certain disadvantages, of which the increased demand for food is 

 the most obvious, which tend to offset the advantages it gains by the 

 reduction of its surface area proportionately to its weight. It is difficult 

 to show that it and its descendants will tend to be preserved by natural 

 selection, relatively to somewhat smaller forms. The increased potential 

 length of life and of the reproductive period is perhaps balanced by the 

 longer immaturity within which it is probable that much of the racial 

 mortality occurs. Thus the history of the horse which appears to provide 

 an admirable case of steady adaptation of a phyletic line to a definite mode 

 of life may perhaps show no more than the internal adaptations which are 

 necessary to enable a large animal to function as well as, but no better 

 than, its small ancestor. 



There are, however, a few cases where we are, I think, on firmer ground. 

 The slow and steady improvements in limb structure which go on in the 

 mammal-like reptiles from Lower Permian to Lower Triassic times take 

 place in animals which do not exhibit a steady increase in size, which 

 indeed cover nearly the same range of sizes at the beginning and end of 

 the story. 



