852 



TIT.'USfOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



OsBORX, Henry Fairfield — Continued. 



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1909.1. See Pearl, Raymond. 



SECTION 2. NATURAL SELECTION IN MAMMALS; 

 CAUSES OF THE EXTINCTION OF THE TITANO- 

 THERES AND OTHER QUADRUPEDS 



In section 1 of this chapter we have treated the 

 origin and evolution of single biocharacters, studied in 

 the light of the Lamarckian, the Darwinian (through 

 natural selection), and the tetrakinetic theories. In 

 section 2 we shall consider the mammals as a whole, 

 inquire as to the causes of their survival and extinc- 

 tion, and determine how far these processes are 

 attributable to variations and fluctuations in single 

 biocharacters and how far to the combinations of 

 biocharacters that make up organisms. 



EXTINCTION OF FAUNAS IN THE AGE OF MAMMALS 



The sudden extinction of the titanotheres is one of 

 the most striking phenomena of the age of mammals; 

 it has generally been dismissed in a few words, but it 

 is by no means so simple as it at first appears, because 

 it seems to call into question ail the laws of natural 

 selection and the relation of these laws to various 

 degrees of fitness or adaptation. 



Among the more general questions of titanothere 

 family evolution are (1) causes favoring increase in 

 size, increase in specialization, and multiplication of 

 the titanotheres; (2) causes bringing about diminution 

 in size, arrest of specialization, diminution of number, 

 and elimination of the phyla; (.3) causes terminating 

 in the extinction of the family. 



The great law of mammalian adaptation through 

 the elimination of the least adaptive becomes less 

 sweeping in its effects as geologic time advances and 

 the Mammalia become more highly perfected. Thus 

 extinction is neither on the same grand zoologic scale 

 nor chiefly due to the same causes through the succes- 

 sive geologic epochs of the Tertiary period. During 

 the middle of the Eocene epoch extinction is chiefly 

 that of inadaptive orders of archaic mammals; in the 

 late Eocene and through Oligocene time extinction is 



