CAUSES OF THE EVOI;XJTION AND EXTINCTION OF THE TITANOTHERES 



859 



visited the region again and again, at preceding periods. Every 

 plant that was incapable of resisting such a degree of cold, every 

 animal which was exposed to be entirely cut off by an epidemic 

 or by famine caused by the consumption of vegetation by the 

 locusts, may have perished already, so that the subsequ'int 

 recurrence of similar catastrophes is attended only by a 

 temporary change. 



As a geologist Lyell believed that the destructive 

 influence of geologic and physiographic changes was 

 extremely gradual. In 1863 (1863.7, p. 374) he 

 observed : 



It is probable that causes more general and powerful than the 

 agency of man, alterations in climate, variations in the range 

 of many species of animals, vertebrate and invertebrate, and of 

 plants, geographical changes in the height, depth, and extent of 

 land and sea, some or all of these combined, have given rise, in a 

 vast series of years, to the annihilation not only of many large 

 Mammalia but to the disappearance of the Cyrena fluminalis, 

 once common in the rivers of Europe, and to the different range 

 or relative abundance of other shells which we find in the 

 European drifts. 



Darwin. — The next great student of this subject was 

 Charles Darwin. His voyage around the world 

 (1831-1836) on the exploring ship Beagle (1839.1) 

 afforded him an extraordinary opportunity of testing 

 the theories advanced in Ly ell's "Principles" as first 

 published in the previous year (1831). Especially in 

 the Pampean and Patagonian regions he contrasts the 

 BufTon-Cuvier and the Lyell hypotheses in favor of the 

 latter. He dismisses catastrophic causes and in 

 general attributes extinction to a cessation of those 

 world-wide conditions of life which were favorable to 

 the larger quadrupeds in Europe, Asia, Australia, 

 North and South America. In South America and 

 elsewhere (1) he does not favor the extreme theory of 

 the destructive influence of the glacial epoch, and he 

 cites the supposed postglacial survival of Macrau- 

 chenia and Mastodon. "It could hardly have been a 

 change of temperature," he observes (p. 170), "which 

 at about the same time destroyed the inhabitants of 

 tropical, temperate, and Arctic latitudes on both sides 

 of the globe." (2) He dismisses the possibility of 

 extinction by man; (3) also of an extended drought 

 in South America, calling attention to the Pampean 

 horse as an animal which could have survived a 

 drought. 



In seeking to establish a general law of extinction 

 Darwin makes the following propositions: (1) Ani- 

 mals naturally increase in geometrical ratio; (2) the 

 food supply, however, remains constant; (3) any 

 great increase in numbers is thus impossible and must 

 by some means be checked; (4) we are seldom able 

 to state the cause of this check beyond saying that it 

 is determined by some slight difference in climate, 

 food, or the number of enemies; (5) we are therefore 

 driven to the conclusion that causes generally quite 

 inappreciable by us determine whether a given species 

 shall be abundant or scanty in numbers; (6) compara- 

 tive rarity is the plainest evidence of less favorable 



conditions of existence; (7) rarity frequently precedes 

 extinction, and if the too rapid increase of species, 

 even the most favored, is steadily checked, why 

 should we feel such great astonishment at the rarity 

 being carried a step farther to extinction? 



Darwin's earlier views (1839-1845), developed 

 from observations made during his voyage, on droughts, 

 floods, insect life, epidemics, were more fully elab- 

 orated in connection with the publication of his 

 theory of natural selection (1858, 1859). In his 

 "Origin of species" (1859) he discusses more fully the 

 checks to increase as follows: (1) Climate as directly 

 unfavorable; (2) climate as indirectly unfavorable by 

 favoring other forms or by increasing the number of 

 certain competitors; (3) unchecked increase frequently 

 followed by epidemics, possibly in part by facility of 

 diffusion of parasites among the crowded animals; 

 (4) finally, since a large stock of individuals, relatively 

 to the number of enemies, is absolutely necessary for 

 the preservation of a species, a diminished number 

 would tend to extinction; (5) any form (p. 133) that is 

 represented by few individuals will run a good chance 

 of utter extinction during great fluctuations in the 

 nature of the seasons or from a temporary increase in 

 the number of its enemies; (6) diminution in number 

 presents less opportunity for producing favorable 

 variations, hence rare species will be less quickly 

 modified or improved within any given period. 



Wallace. — Wallace in his long series of contributions 

 .to the natural selection and Darwinian theory devel- 

 oped similar views (1858.1). In "Natural selection" 

 (1870.1, p. 14) he observes: 



To discover how the extinct species have from time to time 

 been replaced by new ones down to the very latest geological 

 period is the most difficult and at the same time the most 

 interesting problem in the natural history of the earth. * * * 

 Whenever the physical or organic conditions change, to however 

 small an extent, some corresponding change will be jDroduced 

 in the flora and fauna, since, considering the severe struggle for 

 existence and the complex relations of the various organisms, 

 it is hardly possible that the change should not be beneficial 

 to some species and hurtful to others. 



The majority of the speculations of these great 

 naturalists are abundantly confirmed by modern 

 paleontology. The lines of thought and investiga- 

 tion developed by Lyell and Darwin are precisely 

 those which we are pursuing to-day, but our greatly 

 expanded knowledge of paleontologic history, of the 

 biologic results of physiographic revolutions, of rela- 

 tively sudden extinctions at the close of certain 

 periods, of the means of the spread of epidemics by 

 insect agencies, of the elimination of certain structural 

 types opens up entirely new fields of actual observa- 

 tion and comparison for the formation of more precise 

 and definite conclusions. In short, we are in a posi- 

 tion to substitute for the ingenious and profound 

 speculations of Lyell and Darwin a number of con- 

 crete examples of extinction that can certainly be 

 traced to definite and specific causes. 



