CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



875 



independent local physiographic changes. Early theo- 

 retic observations on this principle are those of Lyell 

 and Darwin. Lyell observes (1872.1, vol. 2, p. 451): 



Extension of the range of one species alters that of others. 

 In reference to the extinction of species it is important to bear 

 in mind that when any region is stocked with as great a variety 

 of animals and plants as its productive powers will enable it 

 to support, the addition of any new species to the permanent 

 numerical increase of one previously established must always 

 be attended either by the local extermination or the numerical 

 decrease of some others. 



Wallace observes: "There is good reason to believe 

 that the most effective agent in the extinction of 

 species is the pressure of other species, whether as 

 enemies or merely as competitors. " Darwin enter- 

 tained similar views. 



The evidence afforded by paleontology does not 

 fully support the sweeping statement made by Wal- 

 lace, for paleontologic research indicates that compe- 

 tition is chiefly between phyla. The conclusion 

 drawn from these exceptions is similar to that of 

 Darwin — that the keenest competitors are animals of 

 most nearly similar feeding habits. 



There are, however, exceptions to Darwin's conclu- 

 sion also, as several examples to be cited demonstrate. 

 The extinction of the titanotheres and elotheres may 

 have been due entirely to changes in vegetation rather 

 than to competition with other Herbivora. Again, 

 the survival of the opossums (didelphids) in North 

 America is a striking instance of successful competition 

 of a very generalized type of marsupial with numerous 

 other small Carnivora by the adoption of nocturnal 

 habits. 



Competition hetween placentals and marsupials. — 

 The gradual introduction of placentals into Australia 

 has recently been summarized by Lucas and Le Souef 

 (Lucas, A. H. S., 1909.1, p. 4). The rodents probably 

 became denizens of the continent in later Tertiary 

 time. They probably came from Asia, for there are 

 no groups related to the peculiar genera of South 

 America and the Cape. The rodents all belong to the 

 widely distributed family of the Muridae. The most 

 specialized genus is Conilurus (Hapalotis), comprising 

 graceful little rats, which take the place of the leaping 

 jerboas of Africa and Asia. Then came in the dingo, 

 a wolf from eastern Asia. Lastly, introduced directly 

 from Europe, came a disastrous and prolific popula- 

 tion of black and brown rats, the common mouse, the 

 rabbit and the hare, and, worst of all, the fox. Dur- 

 ing the year 1908 no fewer than 18 million rabbit skins 

 passed through the Sydney market, besides a nearly 

 equal number through the Melbourne market, and 

 vast numbers of rabbits were exported in cold storage, 

 but these inroads produced httle effectin exterminating 

 the pest. 



Baldwin Spencer observes (1896.1, p. 127) that the 

 existing marsupials are severely handicapped when in 



competition with the rodents by having to carry 

 their yoimg in the pouch; at the age when a young 

 marsupial at sight of danger at once flies to its moth- 

 er's pouch, a young rat or rabbit is taking care of itself. 

 If a hawk or eagle catches the mother rabbit the yoxmg 

 one is left, or vice versa. The marsupial mother 

 has to carry the young ones, and not only does the 

 extra weight prevent her from gaining shelter, but if 

 taught both she and the young ones are sacrificed. 

 A very slight difference in speed will save or lose an 

 animal's life. When hard pressed a kangaroo will 

 throw the young out of the pouch so as to travel 

 faster. The same author (idem, pp. 55-57) in con- 

 sidering the causes of the extinction of the giant 

 Pleistocene kangaroos attributes it to overdevelopment 

 in size during a humid period followed by strenuous 

 competition of food during an arid period. 



Reduction of food supply by smaller Herhivora. — The 

 great changes in the life of the countries encirclirfg the 

 Mediterranean are attributable in part to changes of 

 climate, in part to the destructive agency of man, 

 and in part to the destructive agency of animals 

 introduced and protected by man. The change 

 both in soil and vegetation has been caused indirectly 

 by deforestation of the hills and moimtains, and 

 this has been largely due to the tmrestricted browsing 

 of large herds of sheep and goats, which has been going 

 on since long before the Christian era. Even now 

 goats may be seen in certain parts of Palestine and 

 Greece destroying the last of the forests by killing 

 the seedling trees. The destruction of the forests 

 leads to the washing away of the soil, imfitting a 

 country to support any of the larger Herbivora. 

 Man has played so large a part in this disturbance of 

 the natiu-al order that it is hazardous to use these 

 illustrations as analogous to natural conditions in the 

 distant past. 



The competition of the smaller Herbivora, especially 

 on islands, is one which, although by no means demon- 

 strated, is a possible cause of the extinction of the 

 larger Herbivora in past time. For example, in 

 South Dakota and Nebraska the small browsers, 

 such as the oreodonts and horses, which swarmed in 

 herds in lower Oligocene time, may possibly have 

 affected the food supply of the large titanotheres. 

 In Oligocene Europe, similai'ly, the great multipli- 

 cation of the small browsers, known as cenotheres, 

 may have been prejudicial to the larger mammals. 



The introduction of new forms of browsing and 

 grazing animals may in certain periods have disturbed 

 the balance of nature and altered the character and 

 amount of food supply and even the water supply and 

 set up new forms of animal competition in certain 

 regions. The placental rabbits to-day certainly 

 exert a great influence on the natural food supply of 

 the marsupial Herbivora of Australia. Cases of 

 overmultiplication are rare in nature, yet not un- 



