884 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



utility justifies may be Icnown as dominant organs. 

 They violate the St. Hilaire law of economy of growth 

 and compensation of parts, or the highest combination 

 of adaptable structure through the subservience of 

 each organ to all the other organs. The great horns 

 of the titanotheres, the giant horns of the Irish deer 

 (Ceruus megaceros), the single tooth of the narwhal 

 (Monodon)', the giant tusks of the elephants are 

 examples of dominant organs. 



The very development of such organs has long been 

 considered one of the possible causes of extinction, 

 We immediately observe that certain dominant organs, 

 like the tusks of the elephant, serve a great variety 

 of useful purposes; they are by no means developed 

 solely for service in the struggle between the bulls for 

 possession of the females. Again, if large horns are 

 considered fatal, it is noteworthy that the small- 

 horned titanotheres {Menodus, Brontops) disappeared 

 simultaneously with the long-horned titanotheres 

 (Brontotherium and Megacerops). Were it not for 

 this fact we might have attributed the extinction of 

 the titanotheres to the possession of such apparently 

 useless dominant organs as their horns. 



SELECTION OF SEXUALLY DOMINANT ORGANS 



The general explanation that has been offered for 

 certain dominant characters peculiar to the males 

 is that they have been developed or perhaps over- 

 developed, through the competition of the males for 

 the possession of the females. Thus, in respect to 

 horns, incisor and canine tusks there has been an 

 incidence of selection on organs that were employed 

 for combat between males and that were of Httle or 

 no use at other times except in standing off carnivorous 

 enemies. 



Noteworthy facts derived from our study of the 

 titanotheres are that the horns first arise alike in 

 both sexes as rudiments, or extremely small horns; 

 that they appear to be equally developed in the males 

 and the females; and that they gradually become 

 distinctively male characters, so that the sexes are 

 sharply separated by the size and development of the 

 horns. The horns were undoubtedly of advantage 

 to the males in their sexual combats for the possession 

 of the females, and a constant selection of individuals 

 with the largest horns may have been in process. 

 This main emphasis of natural selection on characters 

 which are useful for competitive and combative 

 purposes, it is argued, may have been the cause of the 

 arrested evolution of the grinding teeth. 



It is certainly true that the grinding teeth attained 

 a higher mechanical perfection in the short-horned 

 titanotheres than in the long-horned brontotheres, 

 and this would serve to prove that in the menodonts 

 the teeth were being favored by selection, while in the 

 brontotheres the horns were being favored by selection. 

 As noted above, the superiority of the menodonts in 



the matter of tooth structure did not save them from 

 the extinction which overtook the brontotheres. 



CAUSES OF OVERDEVELOPMENT 



Naturalists generally have attributed overdevelop- 

 ment to the selection of favorable fluctuations in size, 

 but Cope (1896.1, p. 480) invoked a certain kind of 

 intrinsic hereditary force. He says: "I have therefore 

 assumed as a working hypothesis the existence of the 

 bathmic energy and have inquired how far the facts 

 in our possession sustain it." Similarly, the over- 

 development of dominant organs has recently been 

 explained by F. B. Loomis (1905.1, p. 843) as the 

 result of "momentum." After discussing many 

 known instances of organs that appear to have passed 

 a stage of utility, he observes: 



The above are selected examples in which a feature once 

 useful has been developed beyond its maximum utility. Many 

 others equally striking might be cited, the e.xplanation of all of 

 which is extremely difficult unless such a factor as momentum is 

 called in. In the light of this factor, however, a logical and ap- 

 parent cause is found. Momentum also explains why a charac- 

 ter that originated in accordance with the environment develops 

 so rapidly, and why, when an animal had reached adjustment 

 to its surroundings, it still goes on beyond a perfect adjustment. 

 It may be laid down as a rule, then, that a variation started along 

 any line tends to carry that line of development to its ultimate, 

 being driven by momentum. If the feature is detrimental the 

 group dies out; if, however, it is merely a minor feature it makes 

 a handicap. A line of development may be stopped and its 

 momentum overcome, but the tendency is to keep right on. 



RATES OF BREEDING AND EXTINCTION 



It has already been shown that failure or inability 

 to protect the young, or possibly the inhibition of 

 fertihty through low temperature, must be among the 

 causes of extinction, and as extinction rapidly follows 

 when the death rate exceeds the birth rate, the rate 

 of breeding must be considered in any inquiry as to 

 extinction. Arthur Erwin Brown, in a letter written 

 October 24, 1907, has called the attention of the 

 writer to this subject and has contributed some 

 valuable notes. He observes: 



The thought suggests itself that the long breeding period may 

 have been a factor in the decline of the Perissodactyla. In aU 

 perissodactyls gestation is slow. Hodgson 70 years ago gave 

 the Indian rhinoceros 17 to 18 months; it does not appear that 

 these figures have been amended. 



Dr. Frank Baker, late superintendent of the 

 National Zoological Park at Washington, fixed the 

 gestation of Tapirus terrestris at 396 days. The gesta- 

 tion period of such an animal as the tapir, adapted to 

 the long summers and short winters of temperate 

 climates, would naturally be unadapted to the long 

 winters and short summers of the glacial climates of 

 Pleistocene time; consequently the extinction of the 

 Tapiridae in North America during late Pleistocene 

 time may have been due to the exposure of the newly 

 born young to climatic conditions to which they were 

 not adapted. 



