CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



887 



Eocene. The practically universal increase betokens 

 in general the long continuance of favorable condi- 

 tions of life, which seem to have affected all lines 

 of descent. 



Extinction at the "period of maximum growth and 

 size. — Almost without exception extinction in each of 

 the lines of Eocene and Oligocene titanotheres was 

 not preceded by dwarfing but occurred at the period 

 of greatest prosperity and after the completion of a 

 long period of unchecked development. Large size 

 in itself is not a cause of extinction. 



No numerical increase apparent. — Although our data 

 are*far from sufficiently complete to warrant a positive 

 statement, we may say that there is at present no evi- 

 dence of a marked general increase of numbers among 

 the titanotheres such as we observe among other 

 mammalian phyla — the oreodonts and the horses in 

 certain beds, for example. 



Adequate brain capacity. — Although the brain capac- 

 ity of the titanotheres is not so large relatively as that 

 of some existing mammals of similar gize, the brain 

 steadily increases in size as we pass from the Eocene 

 to the Oligocene forms, and there is not sufiicient 

 ground to consider lack of intelligence as a cause of the 

 sudden extinction of these animals. 



Molar tooth structure in relation to longevity. — The 

 direct correlation between the longevity and the fer- 

 tility of the quadrupeds with hypsodontism (elonga- 

 tion of the crowns of the grinding teeth), enabling 

 animals to live a great number of years, has been 

 pointed out. Horses, with their long-crowned teeth, 

 live for 25 years and, foaling every year, would produce 

 22 young; elephants, with their long-crowned teeth, 

 live 90 years and produce several pairs of young. In 

 contrast, such a titanothere as the Eocene Palaeosyops, 

 with its short-crowned teeth, would live for a compar- 

 atively short period and would produce comparatively 

 few yoimg. Theoretically this principle might be one 

 of the means of explaining the early dying out of the 

 broad-skulled genus Palaeosyops, with its short-crowned 

 molar teeth, were it not for the contradictory fact that 

 Manteoceras, also with short-crowned molars, survived, 

 whereas Telmatherium, with relatively long-crowned 

 teeth, apparently became extinct. 



Inadaptation of the grinding teeth. — We have shown 

 that the upper Oligocene titanotheres were apparently 

 making an effort to evolve a hypsodont tooth pattern 

 by the elongation of the outer side (ectoloph) of the 

 superior grinding teeth, but that apparently this effort 

 was futile because of the complete separation of the 

 protocone on the inner side of the tooth. Such a 

 tooth is half hypsodont, half brachyodont, and is 

 obviously inadaptive. 



Apparently the titanotheres reached a cul de sac of 

 evolution in their grinding teeth, and this, on the 

 whole, seems to be the single set of organs in which the 

 titanotheres probably failed to meet the new condi- 



tions of food on the Great Plains. We conclude from 

 the extinction of the large-toothed Menodus ingens that 

 it was not the size of the teeth but the mechanical 

 pattern that was unadapted to the new environment; 

 no further mechanical progress or perfection was 

 possible, hence the cul de sac. 



It is noteworthy that every family of mammals 

 that experimented with teeth of this type (Anoplo- 

 theriidae, Anthracotheriidae, Chalicotheriidae) became 

 extinct sooner or later in Tertiary time. 



There is strong collateral evidence for Kovalevsky's 

 theory that the influence of the teeth is a chief factor 

 in extinction. The only type of mammal with buno- 

 selenodont grinding teeth which survived through 

 Miocene time was the chalicothere, probably because 

 it became a forest-frequenting animal. 



A combination of other causes does not preclude the 

 probability that the inadaptation of the teeth was the 

 chief or leading cause of the numerical reduction and 

 finally of the extinction of the titanotheres. 



CONCLXISIONS EEGAEDING THE THEORY OF NATTJRAI 

 SELECTION OF DARWIN AND WALLACE 



Negative evidence as to minute variation. — The 

 foregoing survey of the evolution of the titanotheres 

 and of the causes of extinction among mammals in 

 past and present time does not appear to support the 

 distinctive feature of Darwin's theory that minute 

 heritable variations (that is, mutations) have sufficient 

 survival value to guide the course of evolution. The 

 chief exceptions to this statement are seen in the very 

 high survival value of immunity to disease and, con- 

 versely, the high eliminating value of nonimmunity, 

 which may possibly appear in some individuals as 

 sudden variations or saltations. 



Degrees of survival value; utility of organs. — Paleon- 

 tology and mammalian zoology now afford positive 

 evidence that the structural and functional inadapta- 

 tion of certain organs to environment has been a 

 primary cause of extinction at all times but chiefly 

 during stress of changes in climate, in physical con- 

 ditions, and in life environment. We observe that 

 certain combinations of mechanical structure and 

 function have finally proved fatal to all mammals 

 possessing them. Thus we are able to state positively 

 that certain single organs of the bony or dental 

 mechanism have had distinct survival or elimination 

 value. 



Combination of organs and adaptability. — There is 

 evidence that in past and present time those animals 

 tend to survive which present the highest adaptive 

 combination of favorable characters, of fully formed 

 organs, the highest adaptability in structure or in 

 habit, the highest potentiality of further evolution 

 j toward new favorable types of habit and of structure. 



Survival value of single organs. — There is evidence 

 I that in phyletic, generic, and family selection, not 



