SOUTH DAKOTA SCHOOL OP MINES 



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question and in the new environment activity may continue 

 as favorable as before. 



In case of actual extinction it is often not possible to 

 ascertain the immediate causes. Sometimes the extinction 

 is due wholly to conditions external to the animals them- 

 selves, such as unfavorable climate, alteration of food sup- 



Figure 17 — Land areas of the world during Oligocene time. A period 

 of continental elevation and reunion followed by the reestablish- 

 ment of connections between the life of the New and Old Worlds. 

 Central Europe submerged or partly archipelagic. African mam- 

 mals and birds partly similar to those of Europe. Madigascar 

 united with Africa. South America entirely separated, its mam- 

 mals developing independently. Australia entirely separated. 

 Closing the Oligocene, another long interval of separation be- 

 tween North America and Europe. Rearranged after W. D. 

 Matthew, 1908. H. P. Osborn. The Age of Mammals in Europe, 

 Asia and North America, 1910. Published by the Macmillan 

 Company. Reprinted by permission. 



ply, ravages of disease, encroachment of hostile species, in- 

 sect pests, et cetera. Again extinction may be due largely 

 to lack of internal adaption or adaptability, for example, 

 the teeth may be fitted for too little variation of food, or the 

 brain may be deficient in size or quality so that the animal 

 lacks resourcefulness, alertness and enterprise. 



