M8 DARWIN AND PALEONTOLOGY 



ers. It is especially noteworthy that in these 

 long-horned phyla the main incidence of selection 

 seems to be diverted to the horns from the teeth 

 which appear to be dwarfed or arrested in evolu- 

 tion. In the short-horned phyla, on the other 

 hand, including one series at least, protected by 

 more slender limbs and more rapid movements, 

 the teeth are constantly sharpened and improved; 

 this may be interpreted as caused by the selection 

 of changes of proportion in the teeth. 



The teeth, however, of all these phyla of 

 titanotheres are of a mechanical type which does 

 not admit of further evolution; they have reached 

 a stage which is a cul-de-sac^ beyond which no 

 progress is possible. The change of environment 

 and of flora, therefore, finds these animals inca- 

 pable of further mechanical betterment either 

 through heredity or through the selection of vari- 

 ations of proportion. All the titanotheres be- 

 come suddenly extinct, and it is noteworthy that 

 all other herbivorous quadrupeds having this cul- 

 de-sac type of grinding tooth also became extinct 

 in North America and in Europe either during 

 the Ohgocene or Miocene periods. 



This is an outhne of the only theoretical inter- 

 pretation which can be offered at present. In it 

 heredity, ontogeny, environment, and selection 

 are supposed to be in continuous interaction or 



" Osborn, H. F. ; " Rise of the Mammalia in North America." 

 Vice-Presidential Address before the American Association for 

 the Advancement of Science. Section of Zoology. Madison, 

 Wis., August 7, 1893. 



