260 EXTERMINATING CAUSES. [CHAP. XIX. 



species had a very wide range, and must therefore have been 

 capable of accommodating themselves to considerable variations 

 of temperature. The same species of megatherium, for example, 

 ranged from Patagonia and the river Plata in South America, 

 between latitudes 31 and 50 south, to corresponding latitudes 

 of the northern continent, and was also an inhabitant of the in 

 termediate country of Brazil, in the caves of which its fossil re 

 mains are met with. The extinct elephant also of Georgia (Ele- 

 yrfias primigenius) has been traced in a fossil state northward 

 from the Altamaha to the Polar regions, and then south westward 

 through Siberia to the south of Europe. 



As to the exterminating causes. I agree with Mr. Darwin, 

 that it is the height of presumption for any geologist to be aston 

 ished that he can not render an account of them. No naturalist 

 can pretend to be so well acquainted with all the circumstances 

 on which the continuance upon the earth of any living species 

 depends, as to be entitled to wonder if it should diminish rapidly 

 in number or geographical range. But if his speculations should 

 embrace a period in which considerable changes in physical geog 

 raphy are known to have occurred, as is the case in North and 

 South America since the megatherium flourished, how much 

 more difficult would it be to appreciate all the effects of local 

 modifications of climate, and changes in the stations of contempo 

 rary animals and plants, on all which, and many other condi 

 tions, the permanence of a species must depend. Until we un 

 derstand the physiological constitutions of organic beings so well 

 that we can explain why an epidemic or contagious disease may 

 rage for months or years, and cut off a large proportion of the 

 living individuals of one species while another is spared, how can 

 we hope to explain why, in the great struggle for existence, some 

 species are multiplying, while others are decreasing in number ? 

 &quot; If,&quot; says Darwin, &quot; two species of the same genus, and of closely 

 allied habits, people the same district, and we can not say why 

 one of them is rare and the other common, what right have we to 

 wonder if the rarer of the two should cease to exist altogether ?&quot; 



In illustration of this principle, I may refer to two beautiful 

 evergreens flourishing in this part of Georgia, species of Gordonia 



