CAUSES OP THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



877 



to maintain their numbers against adverse conditions 

 of pliysical environment, such as extreme cold, Iieat, 

 and drought; of changing food supply; of competition 

 with other quadrupeds; and of epidemics. Of Carniv- 

 ora as destroyers of mammalian life the best recent 

 treatment is that of J. Stevenson-Hamilton (1912.1, 

 pp. 160-162, 355, 357). He says: 



It seems to be one of nature's provisions that the females of 

 most wild animals are, generally speaking, more alert and 

 nervous than the males and therefore, in proportion to their 

 numbers, less frequently fall victims to the stealthy cat tribe 

 than do the solitary members of the other sex. It is thus 

 insured that the destruction of the mothers of the race by 

 natural means shall not exceed the due bounds of economy 

 nor be such as to hamper or retard proper progressive increase. 



Sudden introduction of smaller Carnivora. — T. S. 

 Palmer (1899.1, pp. 93-94) has given a striking sum- 

 mary of the influence of the mongoose: 



The common mongoose of India {Herpestes mungo or H. 

 griseus), * * * a well-known destro3'er of rats, lizards, and 

 snakes, was introduced into Jamaica * * * for the purpose 

 of ridding cane fields of rats. * * * Various remedies were 

 tried, but apparently with little success, until in February, 1872, 

 nine individuals of the mongoose, four males and five females, 

 from India, were introduced. These animals increased with 

 remarkable rapidity and soon spread to all parts of the island, 

 even to the tops of the highest mountains. A decrease in the 

 number of rats was soon noticeable. * * * The mongoose 

 increased, and as the rats diminished, its omnivorous habits 

 became more and more apparent. It destro3-ed young pigs, 

 kids, lambs, kittens, puppies, the native "coney" or capromys, 

 poultry, game, birds which nested on or near the ground, eggs, 

 snakes, ground lizards, frogs, turtles' eggs, and land crabs. It 

 was also known to eat ripe bananas, pineapples, young corn, 

 avocado pears, sweet potatoes, coconuts, and other fruits. 

 Toward the close of the second decade the mongoose, originally 

 considered very beneficial, came to be regarded as the greatest 

 pest ever introduced into the island. Poultry and domesticated 

 animals suffered from its depredations, and the short-tailed 

 capromys {Capromys brachyurus) , which was formerly numerous, 

 became almost extinct in some of the mountainous districts. 

 The ground dove {Columhigallina passerina) and the quail dove 

 {Geotrygon montana) became rare, and the introduced bobwhite, 

 or quail, was almost exterminated. The pecuhar Jamaica petrel 

 {Aestrelata carihhoea) , which nested in the mountains of the 

 island, hkewise became almost exterminated. Snakes, repre- 

 sented by at least 5 species, all harmless, and lizards, including 

 about 20 species, were greatly diminished in numbers. The 

 same thing was true of the land and fresh-water tortoises and 

 the marine turtle {Chelone viridis), which formerly laid its eggs 

 in abundance in the loose sand on the north coast. The destruc- 

 tion of insectivorous birds, snakes, and lizards was followed by 

 an increase in several injurious insects, particularly ticks, which 

 became a serious pest, and a coccid moth, the larvae of which 

 bore into the pimento trees. 



CONTRASTS BETWEEN EXTERNAL (ENVIRONMENTAL) 

 AND INTERNAL CAUSES OF EXTINCTION 



Summarizing the nature and the effect of the exter- 

 nal (environmental) causes of extinction we may note 

 some features in which they contrast with those pecu- 

 liar to extinction from internal causes, namely: 



1. In large part the external causes of extinction 

 originate with cosmic changes — changes in the earth 



itself — such as the elevation or depression of the earth's 

 surface and the extension or contraction of areas of land 

 and water. These changes cause progressive increase 

 of heat or cold imder conditions of either moisture or 

 dryness; progressive increase of moisture or desiccation; 

 and consequent changes of sod, vegetation, forestation, 

 and water supply. From these physiographic and cli- 

 matic changes result the introduction of new compet- 

 itors for food and new enemies, new insect pests and 

 new diseases. 



2. Under changing physiographic conditions, even 

 though extremely gradual, many species and genera 

 have become extinct. Secular desiccation in successive 

 epochs of the Tertiary period, beginning in Oligocene 



. time but intensified chiefly in late Pliocene time, grad- 

 ually became fatal to most of the browsing animals — - 

 indeed, quite as destructive to them as the cold and 

 moisture of the glacial epoch. 



3. Perhaps the most distinctive feature of extinction 

 due to external causes is that it may exterminate the 

 fit and the unfit alike, the adaptive and the inadaptive; 

 it may destroy rather than improve a fauna. The 

 extinction of many forms was certainly a consequence 

 of the extension of glacial ice in North America and in 

 Europe and of the desiccation in Australia. 



4. An equally notable featiire of the external causes 

 of the extinction of a fauna is that they have generally 

 acted locally or in certain regions of the earth's surface 

 only, so that a part of the fauna affected was left to 

 survive elsewhere. For example, the extinction of the 

 horses and the proboscideans in North and South 

 America during Pleistocene time did not prevent their 

 survival in the Old World. 



In contrast with extinction thus arising from 

 external causes is extinction arising primarily from 

 internal causes, such as relative inadaptation, or 

 imfitness, overspecialization, and irreversibility of 

 evolution. These causes have acted simultaneously 

 aU over the world, even imder imiformly favorable 

 climatic conditions as, for example, in the extinction 

 of the great orders of Creodonta, Amblypoda, and 

 Condylarthra during the Eocene epoch. The extinc- 

 tion that originates in internal causes — that is, in 

 relative internal fitness or unfitness — may improve a 

 fauna by eliminating the least-adapted members 

 instead of destroying a fauna. 



INTERNAL CAUSES OF PRESERVATION AND EXTINCTION 



IMMUNITY AND ADAPTATION 



Conditions in Africa. — The existing conditions 

 among the large quadrupeds of Africa are of especial 

 interest because of the increasing evidence that the 

 conditions in North America in Oligocene, Miocene, 

 and Pliocene time are most closely paralleled by those 

 that now prevail in the great upland region of Africa — 

 the central Life belt as distinguished from the coast 

 belt. 



