CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



883 



Coryphodon (lower Eocene), and UintatJierium (upper 

 Eocene)— despite arrested brain development, is one 

 of the most astonishing phenomena of Eocene mam- 

 malian life. Their survival may be accounted for by 

 the development of very effective defensive weapons, 

 the tusks and horns wherewith they stood off their 

 enemies and protected their young. The final extinc- 

 tion of these mammals may be attributed to two 

 causes — low brain power, which may have inhibited 

 the proper defense and care of the young, and the 

 arrested evolution of the grinding teeth, which were 

 actually no larger and little more effective for the 

 comminution of food in the giant Dinocerata of upper 

 Eocene time than in the smaller Coryphodon of lower 

 Eocene time. 



In a race that especially develops tusks and horns, 

 both probably favored by sexual selection, the grind- 

 ing teeth tend to show arrested evolution. The 

 disappearance of the small phenacodonts long prior 

 to that of the large Amblypoda is another illustration 

 of the fallacy of the widespread belief that bulky 

 animals tend to disappear first. 



The competition of the archaic small-brained 

 creodont Carnivora with the diminutive large-brained 

 pro-Carnivora in Eocene time may be only remotely 

 compared with the extinction of the Tasmanian 

 wolf (Thylacinus) and Tasmanian "devil" (Sarco- 

 pMlus) through the introduction of the true dog 

 {Canis dingo) on the Australian mainland. The 

 steady increase in size of the creodont carnivores, as 

 displayed in Patriqfelis and the enormously powerful 

 Harpagolestes, may be placed parallel with the increas- 

 ing size of the equally small-brained Amblypoda. 



Exceptions: large-brained types perish. — Against this 

 strongly cumulative evidence that the brain is one of 

 the single organs whose development has been a deci- 

 sive factor in the preservation or extinction of races of 

 animals we must place certain exceptions. The extinct 

 short-limbed rhinoceros (Teleoceras) of the upper Mio- 

 cene evidently had a larger brain than the surviving 

 true rhinoceros. Again, it appears from intracranial 

 casts that the Pleistocene mastodon of North America 

 had a brain fully as large as that of the existing ele- 

 phants. In both Teleoceras and the mastodon great 

 cerebral development failed to preserve the race. Con- 

 versely, certain very small-brained animals, notably 

 the North American opossums (Didelphiidae) and the 

 Insectivora, have survived, even to the present time. 

 The interpretation put upon these exceptions is that a 

 large cerebral development may be insufficient to 

 compensate for the possession of certain disadvanta- 

 geous organs. 



INADAPTATION OF EXTREME SPECIALIZATION 



Extreme specialization may constitute a chain of 

 causes that lead to extinction. An animal may 

 specialize to an extraordinary degree in a single mode 

 101959— 29— VOL 2 13 



of subsistence, and the food upon which it subsists may 

 be diminished or destroyed ; or a race of animals may 

 expend its energy largely in the development of certain 

 single organs, such as horns or tusks, which become 

 dominant and interfere with the proper development 

 of other organs. If a race undergoes a marked de- 

 generation or loss of organs it can not retrace its steps, 

 because, to use DoUo's dictum, evolution is irrever- 

 sible — that is, lost parts can not be regained. 



SURVIVAL OF THE UNSPECIALIZED 



The extinction of the highly specialized has been so 

 frequently observed that it was formulated into a law 

 by Cope (1896.1, p. 173): 



Agassiz and Dana pointed out this fact in taxonomy, and 

 I expressed it as an evolutionary law under the name of the 

 "doctrine of the unspecialized." This describes the fact that 

 the highly developed or specialized types of one geologic period 

 have not been the parents of the types of succeeding periods, 

 but that the descent has been derived from the less specialized 

 of preceding ages. No better example of this law can be 

 found than man himself, who preserves in his general structure 

 the type that was prevalent during the Eocene period, adding 

 thereto his superior brain structure. 



The validity of this law is due to the fact that the specialized 

 types of all periods have been generally incapable of adaptation 

 to the changed conditions which characterized the advent of 

 new periods. Changes of climate and food consequent on 

 disturbances of the earth's crust have rendered existence 

 impossible to many plants and animals and have rendered life 

 precarious to others. Such changes have been often especially 

 severe in their effects on species of large size, which required 

 food in large quantities. The results have been degeneracy or 

 extinction. On the other hand, plants and animals of un- 

 specialized habits have survived. 



IRREVERSIBLE EVOLUTION 



As shown above in the discussion on bunoselenodont 

 teeth, an organ must be capable of development in 

 a new direction to supply the new needs of the 

 organism; if the organ is lost, degenerate, reduced, 

 or mechanically incapable of further development 

 it constitutes a bar to the survival of its possessor 

 (DoUo, 1893.1). For example, a change of vegetation 

 in Oligocene time appeared to favor the animals 

 that were capable of cropping their food as well as 

 grinding it up into fine particles with the molar teeth. 

 This change found the titanotheres possessed of 

 roimded, button-like incisor teeth, totally useless for 

 cropping. This loss of useful cropping teeth may not 

 have been compensated for, as it was with the square- 

 lipped grazing rhinoceroses, by the evolution of 

 effective upper and lower Hps. It may be said, 

 therefore, that the titanotheres had lost all power of 

 adaptation to grazing habits through the degeneration, 

 simplification, or absence of their incisor teeth. 



INADAPTATION OF DOMINANT ORGANS 



Organs that have reached a stage of development so 

 extreme as to require a larger share of the sum total 

 of bodily nutrition than their general or apparent 



