240 THE AGE OF MAMMALS 



side of the crown. Such a half-hypsodont, half-brachyodont tooth was 

 not adaptive. 



Some other cause, however, must be sought for the extinction of the 

 titanotheres, because the entelodonts, with teeth still less effective me- 

 chanically, and the chalicotheres, with teeth very similar in pattern to 

 those of the titanotheres, both survived through the Oligocene or even into 

 Pliocene times in certain parts of the world. The obvious conclusion is 

 that the entelodonts and chalicotheres cither enjoyed some compensating 

 adjustment or discovered a level habitat suitable to their needs. 



Useless dominant organs.^ — Another explanation which may be offered 

 of the extinction of the titanotheres is that in two phyla the horns were 

 over-developed, attaining gigantic size and causing an incidence of natural 

 selection on characters which were useful in combat only. Characters 

 which have reached an extreme stage so as to demand a larger share of the 

 sum total of bodily nutrition than their general utility justifies may be 

 known as useless dominant organs; they appear to violate the law of 

 economy of growth, or the most favoraljle combination of characters by 

 the subservience of each part to the whole. 



But the force of this theory as applied to the extinction of the titano- 

 theres is completely negatived by the fact that two of the phyla {Titano- 

 iherium, Megacerops) in which the horns were relatively small became extinct 

 at exactly the same time as the large-horned genera {Brontotherium, Sym- 

 horodon). We are thus compelled to believe that the titanotheres became 

 extinct partly through the inadaptation of their grinding tooth structure to 

 sustain their great bulk in a period of incipient desiccation and of changed 

 conditions of climate and vegetation. 



Diminution of browsing animals. — The Oligocene certainly witnessed 

 a world-wide diminution of the larger types of browsing animals with brachyo- 

 dont teeth and with feet incapable of rapid or cursorial locomotion. There 

 is, on the contrary, an increase in the number of grazing animals, accom- 

 panied by an incipient transformation of brachyodont into hypsodont 

 grinding teeth wherever the tooth pattern admits of such a change, and a 

 general elongation of the feet from mesatipodal into dolichopodal types. 



M idtiplication of smaller browsing and grazing animals. — The multi- 

 plication of the small browsing and grazing animals, such as the oreodonts, 

 in Oligocene times is also to be taken into consideration as a possil^le cause 

 of extinction of the larger types like the titanotheres. The oreodonts cer- 

 tainly existed in enormous numbers, and must have consumed great quan- 

 tities of food. The horses also swarmed in herds during the Oligocene 

 periods in the region of South Dakota and Nebraska. It is possible that 

 these animals may have cut off part of the food supply of the titanotheres. 

 Their influence may be compared to that of the intreduction of large num- 

 bers of sheep and goats into a cattle country. The abundance of new 



' Cope, E. D., Primary Factors of Organic Evolution, 1896, p. 173. 



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