854 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Changes of environment are almost infinitelj^ com- 

 plex. Mammals which from various causes may be 

 perfectly, even exquisiteh" adapted in nine hundred and 

 ninety-nine out of a thousand biocharacters may be 

 suddenly cut off through inadaptation in one remain- 

 ing biocharacter; for example, through nonimmunity 

 to certain infectious diseases (Smith, T., 1912.1) or 

 through deficiency of a single organ. 



with diminution, impoverishment of numbers; teleo- 

 logically, from the standpoint of adaptation, it is the 

 arbiter of the fitness of whole phyla of organisms and 

 their success in reproduction, the crucial test lying 

 in the adaptation or inadaptation of correlated parts 

 of the individual animals composing each phylum 

 or in the collective capacity of the phylum as a whole 

 to maintain itself. 



Figure 758. — Affinities and duration of nine families of Perissodactyla 



Extinction of titanotiicres (dotted). The titanotheres play a part in the early history of the order. Their period of extinction is coincident 

 with that of the supposed aquatic rhinoceroses known as amynodonts and that of the light-limbed cursorial rhinoceroses known as hyraco- 

 donts. These animals possessed the adaptive rhinoceros-like tooth structure, yet they became extinct coincidently with the titano- 

 theres, which were provided with cone-and-crescent grinding teeth. The family of chalicotheres, which had cone-and-crescent teeth like 

 those of the titanotheres, survived until late Pliocene time. 



PHASES OF EXTINCTION 



The process of extinction usually presents three 

 phases, as follows: (1) Numerical diminution, local 

 in its effects; (2) regional extermination or dis- 

 appearance, local in effect; (3) extinction of certain 

 races in all parts of the world. These phases are apt 

 to be cumulative — that is, numerical diminution is 

 apt to lead to regional extermination through a 

 series of causes which may finally lead to world-wide 

 extinction. Geographically extinction may be local, 

 continental, or world-wide; numerically it may begin 



MULTIPLE CAUSES OF EXTINCTION 



The first generalization that will be drawn from the 

 broad comparative study made by the author is that 

 there are as many causes and modes of extinction as 

 there are vulnerable points in the structure, functions, 

 habits, and life development of organisms, and as 

 there are changes and vicissitudes of environment; in 

 other words, the causes of elimination of individuals 

 are almost infinitely varied. Whenever the rate of 

 elimination exceeds the rate of reproduction there 

 follows numerical diminution, which may lead to 



