CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



863 



extermination. For example, a herd of animals may 

 be reduced to the danger point in numbers so that 

 they can no longer protect their young. Bell, former 

 Acting Director of the Geological Survey of Canada, 

 observes that the small surviving herd of woodland 

 bison (Bison hison athabascae Rhoads) of British 

 Columbia, although conserved by the Government, will 

 probably be destroyed gradually through the Idlling 

 of the calves by wolves, the bulls not being sufficiently 

 numerous to protect the calves. 



Diminished herds and inbreeding of hison. — In a 

 paper entitled "Das allmahliche Aussterben des 

 Wisents (Bison honasus Linnaeus) iin Forste von 

 Bjelowjesha," Eugen Biichner (1895.1) gives a detailed 

 history of the bison herd in the Bieloviejsha (or 

 Bialowitza) forest, Province of Grodno, in Lithuania, 

 during the present century. 



A careful stud}' of the breeding habits of the bison in the 

 Bieloviejsha forest and elsewhere leaves no room for doubt that 

 the present slow rate of reproduction is an abnormal condition, 

 and that to it is due the rapid approach of the extinction which 

 is the certain fate of the herd under consideration. This dimin- 

 ished fertility the author regards as a stigma of degeneration 

 caused by inbreeding. * * * Another indication of the 

 degenerate condition of the Bieloviejsha herd is seen in the 

 great excess of bulls, which probably outnumber the cows two 

 to one. This is doubtless a result of inbreeding, for Diising 

 (1884.1) has shown that close inbreeding, like a reduced condi- 

 tion of nutrition, is favorable to the production of an excess of 

 males. * * * Jq conclusion, the author considers that his 

 studies of the history of the Bieloviejsha bison leave scarcely 

 room for doubt that inbreeding is the cause of the final extinc- 

 tion of most large mammals. Inbreeding must begin and lead 

 gradually but certainly to the extinction of a species when it, 

 through any cause, has become so reduced in numbers as to be 

 separated into isolated colonies. 



Numerical diminution of llamas. — The observations 

 of Prichard (1902.1, pp. 132, 189, 255) in Patagonia 

 afford an interesting instance of numerical diminution 

 among the Camelidae. 



Around the lake lay piled the skulls and bones of dead game, 

 guanaco {Lama huanachus) and a few huemules (Furcifer 

 chilensis). These animals come down to live on the lower 

 ground and near unfrozen water during the cold season, and 

 there, when the weather is particularly severe, 'they die in 

 ' crowds. We saw their skeletons, in one or two places literally 

 heaped one upon the other [p. 132]. * * * Again we came 

 upon a second death place of guanaco, which made a scene 

 strange and striking enough. There can not have been less 

 than 500 lying there in positions forced and ungainly as the 

 most ill-taken snapshot photograph could produce. Their long 

 necks were outstretched, the rime of the weather upon their 

 decaying hides, and their bone joints ghstening through the 

 wounds made by the beaks of carrion birds. They had died 

 during the severities of the previous winter and lay literally 

 piled one upon another [p. 189]. * * * xhe meaning of 

 this I gathered from Mr. Ernest Cattle. He told me that in 

 the winter of 1899 enormous numbers of guanaco sought Lake 

 Argentine and died of starvation upon its shores. In the 

 severities of winter they seek drinking places, where there are 

 large masses of water likely to be unfrozen. The few last 

 winters in Patagonia have been so severe as to work great havoc 

 among the herds of guanaco [p. 255]. 



Numerical diminution of Sirenia. — Interesting in- 

 stances of the effects of "cold waves" in a subtropical 

 region are those cited by Bangs (1895.1) regarding 

 the rivers of Florida, especially in the case of the 

 manatee (Manatus manatus). These animals are 

 extremely sensitive to a lowering of temperature. 

 During the winter of 1894-95 there was an unusually 

 cold wave followed by marked niunerical diminution, 

 several of these animals being found ashore dead. 



Deforestation and secular cold. — The main natural 

 causes of deforestation appear to be (1) extreme heat 

 and secular desiccation; (2) periodical fires that destroy 

 the young trees; (3) excessive browsing, which destroys 

 the young trees; (4) excessive cold resulting in pro- 

 longed and deep snow mantling; (5) continuously 

 frozen subsoil or tundra condition; (6) plagues of 

 insects and other forest destroyers. 



After considering the former abundant mammalian 

 life m Alaska, IVTaddren (1904.1, pp. 65-66) sum- 

 marizes his conclusions as follows: 



I. That while remnants of the large Pleistocene mammal 

 herds may have survived down to the Recent period and in 

 some cases their direct descendants, as the musk ox, to the 

 present, most of them became extinct in Alaska with the close 

 of the Pleistocene. 



II. The most rational way of explaining this extinction of 

 animal life is by a gradual changing of the climate from more 

 temperate conditions, permitting a forest vegetation much 

 farther north than now, to the more severe climate of to-day, 

 which, subduing the vegetation and thus reducing the food 

 supply besides directly discomforting the animals themselves, 

 has left only those forms capable of adapting themselves to the 

 Recent conditions surviving in these regions to the present. 



Influence of cold and snow on food supply aiid choice 

 of food. — The death of great numbers of animals 

 from hunger or starvation through the covering of 

 food during the winter under heavy layers of snow is 

 commonly observed among the large herds of some 

 of the domesticated horses and cattle on the western 

 plains. In fact, it is most probable that during the 

 glacial epoch the great winter snow blankets covering 

 the natural food were the chief cause of extinction 

 rather than the actual influence of the cold itself. 



Under these conditions horses are driven to eat 

 food which is very deleterious to them, such as the 

 branches of willows. Under the influence of hunger 

 cattle and sheep will feed eagerly and indiscriminately 

 on plants that may be injurious to them or to their 

 young, as recorded by Chesnut and others in the 

 United States Department of Agriculture. The 

 indirect results of hunger may therefore be quite as 

 effective as actual starvation. 



Animals vary greatly in adaptability to new condi- 

 tions caused by prolonged cold and heavy snowfall. 

 Horses remove snow even to depths of 3 or 4 feet and 

 find food to carry them through the winter, whereas 

 under the same conditions cattle starve. 



Influence of cold during the reproduction period. — 

 Exceptional cold waves or unusually prolonged cold 



