882 



TIT,\NOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



In discussing the brain, especially after referring to 

 the general law of brain growth (that is, evolution) 

 during the Tertiary period, he states: 



To this general law of brain growth two additions may now 

 be made, which, brieflj- stated, are as follows: (I) The brain of 

 a mammal belonging to a vigorous race, fitted for a long survival, 

 is larger than the average brain, of that period, in the same 

 group; (2) the brain of a mammal of a declining race is 

 smaller than the average of its contemporaries of the same 

 group. 



The ratio of brain weight to body weight in recent 

 mammals has been made the subject of an exhaustive 

 investigation by Max Weber (1896.1), who publishes 

 the ratios between brain weight and body weight in 



The chief measure of the brain capacity of extinct 

 mammals is indicated by (1) absolute size and weight 

 of the brain, (2) ratio of brain weight to body weight, 

 (3) development of the cerebral convolutions, (4) 

 proportion between the frontal and parieto-occipital 

 lobes of the cerebrum. 



Exceptions: certain small-hrained types survive. — The 



series of diagrams in Plate XLIX shows the enormous 



contrast between the brains of a number of Eocene 



mammals and those of a number of existing mammals 



of equal size and equal bulk. The contrast is drawn 



between pairs of animals of somewhat similar habits 



of life. Thus the Eocene Arctocyon (A) is contrasted 



with the existing dog (Canis), the Eocene Phena- 



codus (B) is contrasted with the 



domestic pig (Sus), both animals 



possessing bunodont teeth and 



somewhat similar omnivorous 



feeding habits. The Eocene 



Coryphodon (C) is contrasted with 



the existing rhinoceros. Finally 



the Eocene Uintatherium (D) is 



contrasted with the Oligocene 



titanothere Menodus giganteus. 



The brains of the archaic mam- 

 mals shown on the left in Plate 

 XLIX {Arctocyon, Phenacodus, 

 Coryphodon, Uintatherium) are not 

 only extremely small in proportion 

 to the size of the animals but are 

 of a low type; they have large 

 olfactory lobes, small cerebral 

 hemispheres, with small frontal 

 lobes; and they are relatively 

 smooth — that is, they are without 

 convolutions. 



An equally intrinsic inadapta- 

 tion seems to have been the inca- 

 pacity for progressive increase in 

 size of brain in successive geologic 

 Figure 760. — Brain proportions in Eocene perissodactyls, an artiodactyl, and an periods; in other words, the brain 

 amblypod of similar size ^^^ consequently the psychic 



D, coionoceras agresiis: E, Hyrachyus powers appear to be in a Condition 

 of arrested development. The 



A., Coryphodon hamatus; 



Ltmnohyops laiiceps; C, Palaeosyops Tobustu 

 bahdianus; F, Amynodon advenus; G, Eporeodonsocialis. 



thirteen existing orders of mammals. In general, (1) 

 small mammals have relatively larger brains than 

 large mammals; (2) within a natural order brain 

 weight does not increase relatively with body weight; 

 (3) in growing individuals the relative brain weight 

 decreases ; (4) in absolute brain weight man is exceeded 

 only by the Proboscidea and Cetacea, in relative brain 

 weight man is exceeded only by certain of the small 

 mammals; (5) in the very primitive marsupials the 

 ratios of brain weight to body weight vary from 

 1:110 to 1:711 as compared with the Carnivora, 

 in which the ratio varies from 1:100 to 1:546. Brain 

 weight, however, does not register the great disparity 

 in intelligence as a survival factor between the Mar- 

 supialia and the Carnivora fissipedia. 



only exception to this rule is seen in one family of 

 archaic creodonts, the Hyaenodontidae, in which the 

 brain attained considerable size in the surviving mem- 

 bers of lower Oligocene time. 



This limited brain power and arrested brain evolu- 

 tion placed these archaic quadrupeds at a great dis- 

 advantage in competition with the more advanced 

 placental mammals, which suddenly appeared in lower 

 Eocene time. The cursorial Phenacodontidae meas- 

 ured their psychic powers with the cursorial Equidae; 

 the small-brained Creodonta generally competed with 

 the incoming true Carnivora, with their progressive 

 increase in size of brain. 



The long survival and steady increase in size of 

 the clumsy Amblypoda — Pantolambda (basal Eocene), 



