South Dakota School of Mine© 69 



One of the most important among these is the cause of extinc- 

 tion* Why was it that animal groups battling for position in 

 life's long race and gaining for a time supremacy in their field, 

 were in turn obliterated by the contending forces of their en- 

 vironment? 



In attempting an answer it should first be stated that the 

 term may not always be fully understood since extinction is 

 sometimes apparent rather than real. Often one species dwin- 

 dles out of existence into another and occasionally, as in 'the 

 horse, camel and rhinoceros, and other families, the consecutive 

 changes may be traced through a long continued series of re- 

 placements by the process of gradual development. Again the 

 seeming extinction may be only a migration from the locality 

 in question and in the new environment activity may continue 

 as favorable as before. 



In case of actual extinction it is often not possible to ascer- 

 tain the immediate conditions. Sometimes the extinction is due 

 wholly to causes external to the animals themselves, such as un- 

 favorable climate, alteration of food supply, ravages of disease, 

 encroachment of hostile species, insect pests, et cetera. Again 

 extinction may be due largely to lack of internal adaptation and 

 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 enter- 

 prise. 



All animal groups pass through innumerable vicissitudes, 

 the immediate effects of which so far as concerns individuals are 

 often harmful, although the result upon the group may be bene- 

 ficial. Too great repetition or abnormal character of the op- 

 pressive conditions tend, always, to deterioration and may in the 

 end lead to obliteration. Of the animals referred to in this 

 japer, several groups are wholly extinct, no relatives of any 

 reasonable nearness being found living today. Notable among 

 such are the Titanotheres and the Oreodons. Reference to the ex- 

 tinction or relationship of others is. given in connection with 

 their description. 



Concerning migration little may be said. Suffice it to state 

 that at certain times new forms appeared from outside regions 

 and broad comprehensive study with reference to both the new 

 and the old world has revealed fairly definite physiographic con- 



*For an excellent recent discussion of this subject see Osborn, 

 H. F. The Causes of Extinction of Mammalia. Am. Nat. Vol. 40, 

 1906, pp. 769-795, and pp. 829-859. 



