Xo. 479] 



EXTINCTION OF MAMMALIA 



773 



in number presents less opportunity for producing favorable vari- 

 ations — hence rare species will be less quickly modified or im- 

 proved within any given period. 



Alfred Russell Wallace 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 species have from time to time been replaced by new ones down 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."^ Also : " Whenever the physical or organic conditions 

 change to however small an extent, some corresponding change will 

 be produced 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 these speculations of these great naturalists 

 have been abundantly confirmed. The opinions of many sub- 

 sequent writers on this subject may be stated under their proper 

 headings. 



ExTERXAL Causes of Extinction 

 Changes 



We may first consider those causes of extinction which originate 

 with changes in the environment. 



Changes of Land Masses and their Connections 

 Changes of land masses caused by elevation or subsidence 

 operate indirectly through causing changes in all the physical 

 conditions of climate, moisture, or dessication, temperature, etc.; 

 also more directly in facilitating or cutting off migrations, in intro- 

 ducing new competition, etc. 



Diminishrd or Contrarfrd Land Arm..— The slahle continents, 

 Xortli America and Africa, underwent slight lluctnations of land 

 area in Tcrtiarv times as .-omparcd with the hiohly unstable conti- 

 nents of Europe, of Australia, an.l of th.- s„uthcrn half of South 



» Wallace, Alfred II . Xnlural ,SV/, rliou . p. 1 l.^ 



