CENOZOIC ERA: AGE OF MAMMALS 603 



The feet of the carnivores seldom show a reduction in the number 

 of toes. This is due to the fact that, since the foot must be adapted 

 for both rending and tearing, as well as for locomotion over both 

 rough and smooth ground, it would not be of advantage to the animal 

 to have the number of toes greatly reduced. The principal changes 

 from the primitive carnivores (creodonts) (Fig. 537) to the modern 

 forms, as far as foot structure is concerned, have been in the perfec- 

 tion of the joints, in the more digitigrade habit of walking (walking 

 on the toes), and in the formation of sharp, retractile claws. 



Limits to Evolution. — There is a limit to evolution after funda- 

 mental modifications in the structure have occurred. For example, 

 thus far no mammal is known to have been transformed from an 

 aquatic to a land type, although numerous examples of the reverse are 

 known. No swift-moving types have retrogressed into slow-moving 

 forms. Animals adapted to tree life, however, are believed to have 

 taken on terrestrial habits and to have become modified to fit them. 



Lost parts are never reacquired ; as, for example, if the number of 

 toes of an animal is reduced, its descendants never have more than 

 the minimum number possessed by its ancestors. Each part that 

 is lost, such as a tooth or a digit, narrows down the possibility of 

 future changes in structure to meet new conditions. A specialized 

 organ can never again become generalized. It will readily be de- 

 duced from the above that animals highly specialized to meet cer- 

 tain conditions will be more likely to fail to meet changed conditions 

 than those that have a more generalized structure. 



REFERENCES FOR THE EVOLUTION OF MAMMALS 



Osborn, H. F., — Age of Mammals, pp. 18-35. 



Scott, W. B., — A History of the Land Mammals in the Western Hemisphere, pp. 645- 

 6 5S . 



Odd-toed Mammals (Perissidactyls) 



This division of the mammals was in the past more important 

 than at present, being now represented by the elephant, tapir, and 

 horse. 



Titanotheres (Greek, titan, a giant, and therion, 2l beast). — Of the 

 many families that, for a time, gave promise of permanence and 

 later became extinct, none is more interesting than the titanothere, 

 a tribe distantly related to the rhinoceros, which is first known in 



