CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



853 



preeminently that of inadaptive families. Miocene 

 time completes the elimination of inadaptive families 

 and is broadly characterized by the extinction of in- 

 adaptive genera, a process which continues through- 

 out Pliocene time. The especial feature of Pleistocene 

 extinction is the ruthless and world-wide regional 

 extermination (as distinguished from extinction) of 

 certain highly adaptive mammals, including both 

 genera and species, over great regions of the world. 



Eocene extinction of orders appears to be due chiefly 

 to the competition between lower and higher orders of 

 mammals — that is, between mammals respectively 

 more or less adaptive in intelligence and in skeletal 

 and dental structure. In Oligocene time there 



a dying tree, decay in one branch after another and 

 finally disappear entirely; or a very flourishing order 

 like the Perissodactyla of lower Oligocene time, with 

 its nine or more families, may be cut down to three 

 families in a comparatively brief geologic period, and 

 its world-wide geographic distribution may narrow 

 down tb a few favorable regions. (See fig. 757.) 



EXTINCTION OF BOTH THE ADAPTIVE AND THE 

 INADAPTIVE 



Some obviously inadaptive zoologic branches, such 

 as the titanotheres, may be cut off, and again some 

 highly adaptive and resourceful animals, such as the 

 horses of the Pleistocene, may yield to new dangers. 



Figure 757. — Extinction of archaic mammals 



Titanothere epoch dotted. Struggle for existence between the archaic (solid black) and modernized (outline) kinds of clawed and hoofed 

 mammals. The surviving insectivores and marsupials are the only remnants of the archaic type. 



occurred more distinctly the first fatal effects of the 

 inadaptation of cjuadrupeds to environmental changes 

 in living conditions, especially to changes in plant 

 life. In Miocene and Pliocene time there occurred as 

 a cumulative process the survival of the forms best 

 adapted to the flora of the open and increasingly arid 

 regions of the plains and uplands. In Pleistocene 

 time all the direct and incidental effects of cold secular 

 changes of climate appear to be the chief causes of 

 extinction. 



GRADUAL OR SUDDEN EXTINCTION 



Even in immense periods of geologic time, extinction, 

 preceded by regional extermination, may be extremely 

 gradual or it may appear to be sudden or even cataclys- 

 mal, as it seems to have been with the titanotheres. A 

 once flourishing order like the Amblypoda may, like 



Adaptation is constantly varying in kind. Among the 

 living races of Africa, for example, a continent which 

 to-day gives us the closest parallel with Tertiary con- 

 ditions of North America, we observe that elimination 

 is constantly standardizmg mammals through destruc- 

 tion of the least adaptive individuals and by this 

 means is keeping the race up to the highest contem- 

 porary plane. Among extinct races, such as the 

 titanotheres, we observe that elimination is pruning 

 off the most specialized phyla of all grades, especially 

 those which exhibit the most extreme specialization. 

 Each of these geologic phyla was subjected to com- 

 petition as a whole with other phyla, which may have 

 evolved contemporaneously; and elimination is the 

 death penalty of the relative inadaptation of a phylum 

 as a whole, inadaptation either to present or to chang- 

 ing environment. 



