856 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



influence has natural selection on the origin and trans- 

 formation of biocharacters, on rectigradation and pro- 

 portion? What degrees of heritable variation in single 

 characters hare a sin-vival value that can be proved by 

 observation or experiment? 



To answer this question finally it is necessary, first, 

 to consider the laws of survival and elimination acting 

 upon quadrupeds as a whole. These laws have been 

 made the subject of a long special investigation by 

 the author for this monograph, the results of which 

 were published in 1906 in a paper entitled "The 

 causes of extinction of mammals" (1906.287). Later 

 results were published in 1910 in "The age of 

 mammals in Europe and North America" (1910.346). 



HISTORY OF OPINION 



Although the mam trend of the present inquiry as 

 to the environmental and the internal causes of ex- 

 tinction had been suggested by the middle of the nine- 

 teenth century, discovery and observation since 

 LyeU's and Darwin's time furnish new material for 

 induction both as to the environmental and the 

 internal causes of extinction. The true method of 

 inquiry is to make close and continuous comparison 

 of the present with past ages, based on the uniformi- 

 tarian doctrines of Lyell and Darwin. Although we 

 know few positive cases of natural extinction in our 

 times, we must study all possible causes of numerical 

 diminution, for we have every reason to believe, with 

 Darwm, that numerical diminution is one of the high- 

 roads to actual extmction. The conception of the 

 similarity of the past and present causes of survival 

 and of the numerous internal causes of variation, 

 development, and decline only gradually took its 

 modern form. 



CATACLYSMAL HYPOTHESKS 



Whewell (1837.1) clearly set forth the history of 

 opinion between 1796, the Buffon-Cuvier period, and 

 1837, the year of the publication of his "History of 

 the inductive sciences." 



Buffon (1749.1), in commenting on the giant extinct 

 fauna of northern Asia and Siberia, pointed out that 

 parts of the globe now submerged were formerly ele- 

 vated; he adumbrated the idea of the separation of 

 faunas, such as the mammoths of Siberia and America, 

 by continental depression, wliich resulted in the sub- 

 mergence of old migration routes. He attributed the 

 disappearance of these animals from the north partly 

 to refrigeration and partly to migration toward the 

 south. 



Cuvier (1825.1) more fully developed the cataclys- 

 mal hypotheses of regional extermination through sub- 

 mergence and through excessive refrigeration. In his 

 "Discours" he observes: 



Let us suppo.se that a great invasion of the sea covers with a 

 mass of sand or other deposits the continent of Austraha; it 



would bury the carcasses of tlie kangaroos, wombats, dasyures, 

 bandicoots, flying phalangers, as well as of the duckbills 

 {Ornithorhynchus) and spiny anteaters {Echidna). It would 

 entirely destroy the species of all these animals, because none 

 of them exist in any other country. 



After advancing the hj^pothesis that Australia thus 

 depopulated might again be populated from Asia, he 

 continues: 



To carry the hypothesis still further, after the Asiatic animals 

 had migrated into Australia let us imagine that a second revo- 

 lution destroyed Asia, the original home of these animals. 



In other words, Cuvier believed in the hypothesis of 

 a total depopulation of a continent through a great 

 and sudden physical revolution. This was in keeping 

 with the cataclysmal geologic theories of the time. 

 He explained the repopulation of continents through 

 migration from other continents in which life had per- 

 sisted. This idea was taken up by Lyell. 



D'Orbigny appeared as a disciple of Cuvier in his 

 travels in South America. He writes: 



I was in a position to study the effects of inundations on the 

 mammals of the province of Moxos (Bolivia), where these 

 inundations are periodical, and I am certain that there the 

 animals instinctively fi}' from the fluvial tide and take refuge 

 in places further removed from the water and on pieces of 

 high ground, where they find themselves momentarily congre- 

 gated together. Ruminants sometimes die for want of food 

 under such circumstances, and the natives mention certain 

 years when this has occurred, but their bodies remain far from 

 the rivers on small plateaux, or in the depths of the forests. 



In summing up his results as to the destruction of 

 the Pampean Pleistocene fauna D'Orbigny (1835.1) 

 says that it is to the sudden rise of the Cordilleras 

 he attributes the sudden movement of the sea, which 

 invaded the continent all at once, carried off and over- 

 whelmed the mastodons which inhabited the eastern 

 flanks of the Bolivian Cordillera, the megatheriums, 

 megalonyxes, and the multitude of animals daily being 

 discovered in the caverns and the fissures of the 

 mountains of Brazil — all the species, in fact, which 

 are extinct. Again he observes: 



My final conclusion from the geological facts I observed in 

 America is that there was a perfect coincidence between the 

 upheaval of the Cordilleras and the destruction of the great 

 race of animals and the great deposit of Pampas mud. 



The stable continents. North America and Africa, 

 underwent moderate fluctuations of land area in the 

 Tertiary period as compared with the highly unstable 

 continents of Europe, Australia, and the southern half 

 of South America. Europe was the scene of alter- 

 nating marine and fresh-water conditions, of varying 

 coast lines, of insular and archipelagic land masses — 

 changes which are all to be more seriously studied in 

 connection with extinction than they have been here- 

 tofore. 



It must be stated at once that the grand phenomena 

 of extinction in unstable Europe from basal Eocene to 

 Pliocene time broadly coincide with those observed in 



