860 



TITAXOTHERES OF ANCIEiSTT WYOMING, DAKOTA, AND NEBRASKA 



ENVIRONMENTAI CAUSES OF EXTINCTION 



THE PHYSICAL EXVIKONMENT 



PHYSIOGKAPHIC CHANGES 



In our inquiry as to the causes of extinction we may 

 first consider those which originate witli clianges in 

 the environment brought about by geographic revo- 

 lutions — such clianges in land masses, including eleva- 

 tion and subsidence, as act directly upon all the physi- 

 cal conditions of climate, moisture, or desiccation, 

 changing vegetation, etc., and facilitating or restrict- 

 ing migration, with resulting new competitions, etc. 



Base-leveling a cause of extinction. — The effects of 

 base-leveling, or the erosion of mountainous and other 

 land surfaces to a single peneplain, has been especially 

 discussed by Woodworth (1894.1) and Adams (1901.7; 

 1904.1). Woodworth points out that the develop- 

 ment of the peneplain, the wideniug of the lowlands, 

 plains, and jungles nearly to tide level in Cretaceous 

 time, was highly favorable to the water-loving Reptilia 

 and imfavorable to mammalian life. New species are 

 brought into competition when a mountain system is 

 leveled off, thus throwing the life of its opposite slopes 

 into the same field. The degradation of uplands has 

 a very direct effect, since organisms suited to steep 

 slopes and high altitudes with low temperatures must 

 migrate, vary, or live at a disadvantage as the surface 

 is lowered by denudation. Other things being equal, 

 the endemic lowland forms will have an advantage 

 over those organisms which are living under the trial 

 of alternating environment with the added stress of 

 contest with hitherto unmet species. Periods of base- 

 leveling are characterized by relative stability of the 

 land with refei'ence to the sea, while periods of glacia- 

 tion are characterized by relative instability. 



Insular conditions. — The substitution of insular for 

 archipelagic or continental conditions by subsidence 

 has undoubtedly been a potent cause both of extermi- 

 nation in certain localities and of the survival of 

 geologically ancient primitive forms (Wallace), such 

 as Monotremata and Marsupialia, in the AustraUan 

 region. It may be said at once, with Lyell, that 

 most of the causes both of survival and extinction 

 which prevail on continents are intensified on islands; 

 yet on islands the phenomena are those of local 

 extermination and modification rather than the 

 general extinction of families and orders, which is our 

 real subject of inquiry. 



Dwarjed Pliocene and Pleistocene island, lije. — In the 

 islands of Malta, Cyprus, and Crete, as explored by 

 Miss Bate (1905.1), we have fine examples of compara- 

 tively recent insulation. 



It appears probable that Cyprus became an island 

 first, because (1) no submerged bank connects it 

 with the mainland, and the 200-fathom line is reached 

 within a short distance of the coast line; (2) the ter- 

 restrial fauna and avifauna include several distinct 



races peculiar to the island, a fact confirmed by 

 Kobelt from his study of the recent Mollusca. The 

 reduced existing Cyprus fauna contains a mingling of 

 European and north African forms and shows the 

 effects of deforestation in historic times. The largest 

 animal on the island is the moufflon {Ovis opMon), 25 

 inches high at the shoulders; yet this is the smallest 

 of all the wild sheep and is related to east Persian 

 species. 



The affinity of Malta to Sicily is indicated by the 

 occurrence of two species, Hippopotamus pentlandi 

 and Elephas mnaidriensis, in the cavern deposits of 

 both islands. The early separation of Cyprus is 

 indicated by the fact that E. Cypriotes and H. minu- 

 tus are both more primitive than the Maltese-Sicilian 

 species. Crete also includes antelope and deer in its 

 Pleistocene fauna. 



Pleistocene extinct fauna of the Mediterranean islands 



Cyprus- 

 Malta __ 



Sicily.-- 

 Sardinia 



Proboscidea (pigmy elephants) 



E. Cypriotes ._ 



E. melitensis, E. mnaidri- 

 ensis. 



E. mnaidriensis 



E. lamormorae 



Artiodactyla (pigmy 

 hippopotami) 



H. minutus. 

 H. pentlandi. 



H. pentlandi. 



The occurrence of these specifically different though 

 apparently closely related races of small elephants and 

 hippopotami in widely separated islands is an instance 

 of independent development with some divergence 

 from common ancestors. 



Volcanic ash. — The comparatively recent recogni- 

 tion that vast areas, both in the Rocky Mountain re- 

 gion and in Patagonia, were the seats of deposition of 

 volcanic dust arising from volcanic eruptions indicates 

 that this may be considered among the means of 

 numerical diminution if not of extermination in certain 

 regions. There is evidence in Patagonia especially 

 that many animals were overtaken and buried entire 

 in volcanic dust; but this is less frequently true of the 

 volcanic dust deposits in the Eocene of North 

 America, where few "entire" burials have been found. 



Floods. — It appears that floods have veiy frequently 

 been a cause of marked diminution in number, which 

 is a very different result from regional extermination 

 or extinction. The instances cited by Howorth, by 

 D'Orbigny, and by Azara (as quoted by Darwin) 

 indicate that floods may be considered among the 

 secondary or contributary causes of extermination. 



In the summer of 1867, it is recorded, over 2,000 

 buffalo out of a herd of about 4,000 lost their lives in 

 the quicksands of the River Platte while attempting 

 to cross (Hornaday, 1889.7, p. 421). ' The floods and 

 swollen rivers of Pleistocene time presented a new set 

 of conditions. 



