INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



27 



Thus out of the nine original famiUes of the great 

 order of Perissodactyla only three — the horses, tapirs, 

 and rhinoceroses — have survived to the present time, 

 and these during the glacial epoch were greatly 

 reduced both in numbers and in geographic dis- 

 tribution. 



The consideration of these facts raises the whole 

 problem of the origin and adaptive radiation of the 

 perissodactyls (see Chap. X) and the general problem 

 of the causes of the extinction of the perissodactyls 

 and of other quadrupeds (see Chap. XI). 



adaptive origin of new characters. The moment of 

 origin of each new character is a very important 

 moment in the history of that character. Does each 

 new character arise fortuitously at this point or that, in 

 an adaptive or inadaptive condition, or does each new 

 character arise in a mechanically adaptive condition, 

 although this condition may be merely incipient? 



The biologic purpose of the long and dry descrip- 

 tions and tables of measurements given in Chapters 

 V, VI, and VII of this monograph is to direct obser- 

 vation continuously to this problem of the origin of 



Figure 20. — Periods of expansion and extinction of the perissodactyls and contemporary forms 



Showing that the expansion of the perissodactyls was coincident with the extinction of the archaic Condylarthra and Amblypoda and that the 

 extinction of many perissodactyls was coincident with the expansion and adaptive radiation of the artiodactyls. 



CAUSES OF EVOLUTION 



There can be no doubt as to the survival value of 

 certain finished types of tooth structure and Hmb 

 structure (see pp. 880-881), a principle first formulated 

 by the distinguished Russian paleontologist Kova- 

 levsky (1873.1). Two important questions that the 

 reader must keep in mind in considering the origin 

 of innumerable new characters are (1) whether 

 there is evidence of chance origins and chance rudi- 

 ments of certain types of structure possessing suffi- 

 cient survival value to establish themselves through 

 the principle of the survival of the fittest, or natural 

 selection; or (2) whether there is some other ortho- 

 genetic principle at work causing the definite and 



new characters. Our general conclusions concerning 

 these two questions are presented in Chapter XI. 



ADAPTIVE EVOLUTION AND OVEEEVOLUTION OF THE 

 FORM OF SKULL, TOOTH, AND FOOT 



Whatever may be the causes of evolution its re- 

 sults are definite. The visible evolution of all the 

 hard parts of the body in herbivorous animals is 

 originally mechanical and manifests general adapta- 

 tion to two broad groups of purposes: 



1. Prehension of food (lips, teeth, and jaws); com- 

 minution of food (teeth and jaws); conservation and 

 transportation of stored food energy (body and limbs). 

 These purposes involve all the mechanical changes of 

 structure of skull and tooth. 



