THEORIES AS TO ORIGIN, ANCESTRY, AND ADAPTIVE RADIATION 



793 



There is some reason to believe that in the combats 

 between the males for the possession of females the 

 horns attained very high sexual selection value, that 

 there was an incessant selection of the large horns at 

 the expense of the selection of other characters, and 

 that this breeding for large horns may have injuriously 

 affected the evolution of the grinding teeth, which is 

 inferior in Brontotherium to the molar-tooth evolution 

 in the short-horned Menodus. This problem is dis- 

 cussed in Chapter XI as among the causes of the 

 extinction of the titanotheres. 



CORRELATION OF HORNS WITH SEX 



Sexual disparity in horn development becomes more 

 marked as we ascend to higher geologic levels. The 



EVOLUTION OF THE BRAIN 



The brain cavity of MesatirJiinus peter soni and of 

 Palaeosyops leidyi, as shown in the intracranial casts 

 (fig. 713), is relatively small. Thus it may be stated 

 that both the Eocene and Oligocene titanotheres — that 

 is, the whole titanothere family — were characterized 

 by a small brain. 



We are indebted to Marsh (1884.1) for a discussion 

 of the brain characters of certain Eocene titanotheres 

 and contemporary Eocene ungulates as compared with 

 those of certain related modernized forms; also for 

 excellent illustrations (fig. 714) reproduced herewith. 

 In the titanotheres (LimnoJiyops laticeps and f Palaeo- 

 syops rolustus) we observe again that the brain is 



Relative size of brain and skull in titauutlieres and other Eocene 

 perissodactyls, an artiodactyl, and an amblypod 



After Marsh, 1884.1. Scales various. A, Coryphoion hamaius; B, Limnohyops laticeps; C, Palaeosyops robuslus; 

 D, Colonoceras agrestis; E, Hyrachyus bairdianus; F, Amynodon advenus; G, Eporeodon socialis 



brontothere females are smaller and wholly different 

 in skull structure from the males; there are also some 

 differences in dental structure which are difficult to 

 comprehend, such as the apparent prominence of the 

 cingulum on the grinding teeth of the females of 

 Brontotherium and its absence on the grinding teeth 

 of the males. Even in the male Brontotherium the 

 canine teeth are relatively small and inoffensive as 

 weapons, as compared with those of Menodus. In the 

 females the canines are greatly reduced in size. This 

 would indicate that, as among the existing horses and 

 cattle, the males stood guard over the herds of titano- 

 theres, protecting the females and the young. 



relatively smaller than in the contemporary Rhino- 

 cerotoidea (Colonoceras, Hyrachyus, and Amynodon). 

 It is also relatively smaller than in the modern Tapirus, 

 Rhinoceros, and Equus cahallus. 



The disparity in the size of the brain of Dolicho- 

 rhinus hyognathus and of the modern Equus cahallus is 

 very striking. Inasmuch as the skulls of these two 

 animals are approximately of the same length, they 

 afford an excellent basis of comparison. The ratio of 

 weight of brain to weight of body in all the other large 

 ungulates has been carefully calculated by Max 

 Weber (1897.1) as follows. 



