2s o. 480] 



EXTINCTION OF MAMMALIA 



845 



Summarizing the external causes we observe: 



(1) That in large part they originate with cosmic changes, or 

 with changes in the earth itself, in the elevation or depression, 

 extension or contraction of the land and water areas. From these 

 result progressive heat or cold under both moist and dry conditions, 

 progressive moisture and desiccation, consequent changes of soil, 

 vegetation, forestation, water supply. Also the introduction of 

 new food competitors or enemies, of new insect pests and new 

 diseases. 



(2) Under these changed conditions we observe that the ex- 

 tinction of species and genera has repeatedly occurred on a very 

 large scale. Secular desiccation in different periods of the Tertiary, 

 but chiefly toward the late Pliocene, was quite as fatal as the 

 Glacial Period. 



(3) A distinctive feature of such extinction, originating in 

 external causes, is that it often affects the fit and unfit alike, the 

 adapted and inadapted; it often destroys rather than improves 

 a fauna. This was certainly the case with the glacial extinction in 

 North America and Europe. 



(4, On tlie contrary, the extinction, nri;/inafiNg in internal 

 cau.sr.s, 1. in niafirr infrrnal [ifnr.ss or un/ifnc^.s, often improves 



(.")) A further distinction is that external causes have usually 

 acted locally or on certain parts of the earth's surface, leaving a 

 part of tlie faima to survive elsewhere. The elimination of the 

 Equidse and Proboscidia in North and South America, for example, 

 did not hinder their survival in the Old World. 



(6) Internal causes, relative inadaptation or unfitness, have, 

 on the contrary, acted simidtaneously all over the world, for exam- 

 ple, in the elimination of the great orders of Creodonta, Ambly- 

 poda, and Coiidylarthra during the Eocene period. 



times hut chiefly during periods of external change. 



