CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



857 



the stable continent of America. This very striking 

 fact shows, first, how cautious we must be in connecting 

 extinction directly with physiographic changes; sec- 

 ond, that extinction from internal causes or through 

 competition with other species is more manifold than 

 extinction through external causes or the struggle with 

 environment. 



Wallace (1876.1) discussed the extinction of the large 

 Pliocene and early Pleistocene mammals of Australia 

 chiefly from the standpoint of a physiographer — that 

 is, he attributed Australian extinction chiefly to the 

 increased competition, or struggle for existence, caused 

 by the progressively contracted land area due to sub- 

 sidence; also to possible glacial conditions. 



The most recent advocate of this now abandoned 

 cataclysmal hypothesis is Howorth. In his interest- 

 ing and encyclopedic work "The mammoth and .the 

 flood" (Howorth, 1887.7, pp. xvii-xviu) he marshals 

 a large number of facts in support of the hypothesis of 

 widely destructive floods both in Asia and South 

 America in Pleistocene time. He observes: 



These facts, I claim, prove several conclusions. Tiiey 

 prove that a very great catastrophe or cataclysm occurred at 

 the close of the Mammoth period, by wMch that animal, with 

 its companions, were overwlielmed over a very large part of the 

 earth's surface. Secondly, that this catastrophe involved a 

 very widespread flood of water, which not only kiUed the ani- 

 mals but also buried them under continuous beds of loam or 

 gravel. Thirdlj', that the same catastrophe was accompanied 

 by a verj' great and sudden change of climate in Siberia, by 

 which the animals wliich had previously Hved in fairly tem- 

 perate conditions were frozen in their flesh under ground and 

 have remained frozen ever since. 



UNIFORMITARIAN THEORIES 

 lYEH ON EXTINCTION 



Development of opinion. — Under the influence of the 

 geologist Hutton the fu'st note for modern methods of 

 explanation and research as to the causes of extinction 

 was struck by Lyell (1830.7). 



Lyell's remarkable discussions of "changes of the 

 organic world now in progress" and their bearing on 

 the phenomena of life in geologic times will be foimd 

 in the first edition of his "Principles of geology" 

 (1830.1, vol. 2), a volume which was dedicated 

 December 8, 1831, and published in January, 1832. 

 Darwin departed on his voyage December 27, 1831, 

 and took Lyell's work with him; he was profoundly 

 influenced by it. 



Lyell himself had been greatly influenced by Buf- 

 fon's theories as to the destructive action of physio- 

 graphic and climatic changes and by the uniformita- 

 rianism of Lamarck; also by Cuvier in respect to the 

 repopulation of devastated land areas by migration, 

 although in other respects he was an archopponent 

 of Cuvier's cataclysmal hypotheses. 



The following citations from Lyell's work, volume 

 2, chapter 10, give a clear insight into his opinions at 

 that time: 



[Centers of creation.] — For we assume, on grounds before 

 stated [chapter 8] that the original stock of each species is intro- 

 duced into one spot of the earth only, and, consequently, no 

 species can be at once indigenous in the Arctic and Antarc- 

 tic circles [p. 170]. * * * The following may, perhaps, 

 be reconcilable with known facts: Each species may have had 

 its origin in a single pair, or individual, where an individual was 

 sufficient, and species may have been created in succession at 

 such times and in such places as to enable them to multiply 

 and endure for an appointed period, and occupy an appointed 

 space on the globe [p. 124]. * * * Now this congregating, 

 in a small space, of .many peculiar species would give an 

 appearance of centers or foci of creation, as they have been 

 termed, as if there were favorite points where the creative 

 energy has been in greater action than in others and where 

 the numbers of pecuhar organic beings have consequently 

 become more considerable [p. 126], 



[Physiographic changes and accommodation.] — Each change in 

 the physical geography of large regions must occasion the 

 extinction of species [p. 158]. * * * Species we have stated 

 are, in general, local, some being confined to extremely small 

 spots and depending for their existence on a combination of 

 causes which, if they are to be met with elsewhere, occur only 

 in some very remote region. Hence it must happen that when 

 the nature of these localities is changed the species will perish; 

 for it win rarely happen that the cause which alters the character 

 of the district will afford new facilities to the species to establish 

 itself elsewhere [p. 166]. * * * if^ therefore, we admit 

 incessant fluctuations in the physical geography, we must, 

 at the same time, concede the successive extinction of terrestrial 

 and aquatic species to be part of the economy of our system 

 [p. 168]. * * * To pursue this train of reasoning farther is 

 unnecessary; the reader has onl}^ to reflect on what we have 

 said of the habitations and the stations of organic beings in 

 general; * * * he will immediately perceive that, amidst 

 the vicissitudes of the earth's surface, species can not be im- 

 mortal but must perish one after the other, like the individuals 

 wliich compose them. There is no possibiUty of escaping from 

 this conclusion, without resorting to some h3-pothesis as violent 

 as that of Lamarck, who imagined, as we have before seen, that 

 species are each of them endowed with indefinite powers of mod- 

 if3dng their organization, in conformity to the endless changes 

 of circumstances to which thej' are exposed [p. 169]. * * * 

 But the power of accommodation to new circumstances is great 

 in certain species and might enable many to pass from one 

 zone to another, if the mean annual heat of the atmosphere 

 and the ocean were greatlj' altered [p. 171]. * * * This 

 argument is applicable not merely to climate, but to any other 

 cause of mutation. However slowly a lake may be converted 

 into a marsh, or a marsh into a meadow, it is evident that 

 before the lacustrine plants can acquire the power of living in 

 marshes, or the marsh plants of living in a less humid soil, 

 other species, already existing in the region and fitted for these 

 several stations, will intrude and keep possession of the ground 

 [p. 174]. * * * ^ faint image of the certain doom of a 

 species less fitted to struggle with some new condition in a 

 region which it previously inhabited and where it has to con- 

 tend with a more vigorous species is presented by the extirpa- 

 tion of savage tribes of men by the advancing colony of some 

 civilized nation. In this case the contest is merely between 

 two different races, each gifted with equal capacities of im- 

 provement — between two varieties, moreover, of a species 

 which exceeds all others in its aptitude to accommodate its 

 habits to the most extraordinary variations of circumstances 

 [p. 175]. 



[Climate.] — Some of the effects which must attend every 

 general alteration of clirhate are sufficiently peculiar to claim a 

 separate consideration before concluding the present chapter 



