No. 479] 



EXTINCTION OF MAMMALIA 



775 



the theory of extinction throntrh the direct action of cold is very 

 sim[)le, })ut it is found that some cases of extinction diirino: the 

 Glacial Period, of the horse in North America for example, do not 

 admit of this explanation. It is more in accord with the facts to 

 say that the Glacial Period ori(?inated certain new conditions of 

 life which hastened extinction; these conditions relate to enforced 

 migration, to overcrowdintr, to feeding, re{)rodiicti()n, mating, re- 

 lations to enemies, deforestation, and other indirect results. 



Profccflvc Adaptafioii to Secular 6W.— Resistance to cold 

 depends upon (a) internal heat-producing powder which is a pro- 

 gressive adaptation of the higher Mammalia, (/>) the acquisition 

 of a warm external covering. The -well known cases of adapta- 



mammoth), rhinoceroses (R. livhorh'nins, woolly rhinoceros), 

 horses (/•:. przciraLs-kii), and certain northern ruminants, sucii as 

 the camels and musk oxen, show that we must not assume that 



Glacial and Post-iUacial Exilurfnni. - Walla-c oh.crvcs: 

 "....We have proof in l.otli Knn.pr a-id Norih Anu-rica, tliat 

 just about the time th('M> lar-r a-iiuia!^ \\(Mv di-a}>|H-ann,u. all the 

 northern parts of these continents w(Mt \vra;)pcd in a mantle of ice; 

 and we have every reason to l,clic\c that the pivx-ncc of ihi^ h.rgc 

 quantity of ice (known to ha\c l.ccn thon>a-uU of fivt if not M.nu- 



duced alrcration. of le^cl of the ocean a^ x\c!i a. xaM local Hood.. 

 whi<-li wonld haM' conilmu.! uith the eM•r..i^e cold to dotroy 

 animal life."^ And again: "... . Wc can thcivfore lianlly fail to 



table life that have occurred in Europe ami North America i)etween 

 the Miocene period and the present day. in part at lea^t, to the two 

 or more cold epochs that hav(> probably int<'iAcncd. The.e 

 chang.>- conM^t. first, in th<M>\iincti..n of a uli,»l<- 1m)M of the hiirher 



Western hemisplicres than bct'orc existed."" 



