l84 CAUSES OF EXTINCTION chap, 



both sides of the globe. In North America we positively know 

 from Mr. Lyell that the large quadrupeds lived subsequently to 

 that period, when boulders were brought into latitudes at which 

 icebergs now never arrive : from conclusive but indirect reasons 

 we may feel sure, that in the southern hemisphere the Macrau- 

 chenia, also, lived long subsequently to the ice-transporting 

 boulder-period. Did man, after his first inroad into South 

 Arnerica, destroy, as has been suggested, the unwieldy Mega- 

 therium and the other Edentata? We must at least look to 

 some other cause for the destruction of the little tucutuco at 

 Bahia Blanca, and of the many fossil mice and other small 

 quadrupeds in Brazil. No one will imagine that a drought, 

 even far severer than those which cause such losses in the 

 provinces of La Plata, could destroy every individual of every 

 species from Southern Patagonia to Behring's Straits. What 

 shall we say of the extinction of the horse ? Did those plains 

 fail of pasture, which have since been overrun by thousands 

 and hundreds of thousands of the descendants of the stock 

 introduced by the Spaniards ? Have the subsequently intro- 

 duced species consumed the food of the great antecedent races ? 

 Can we believe that the Capybara has taken the food of the 

 Tojcodon, the Guanaco of the Macrauchenia, the existing small 

 Edentata of their numerous gigantic prototypes ? Certainly, 

 no fact in the long history of the world is so startling as the 

 wide and repeated exterminations of its inhabitants. 



Nevertheless, if we consider the subject under another 

 point of view, it will appear less perplexing. We do not 

 steadily bear in mind how profoundly ignorant we are of the 

 conditions of existence of every animal ; nor do we always 

 remember that some check is constantly preventing the too 

 rapid increase of every organised being left in a state of nature. 

 The supply of food, on an average, remains constant ; yet the 

 tendency in every animal to increase by propagation is 

 geometrical ; and its surprising effects have nowhere been more 

 astonishingly shown, than in the case of the European animals 

 run wild during the last few centuries in America. Every 

 animal in a state of nature regularly breeds ; yet in a species 

 long established, any great increase in numbers is obviously 

 impossible, and must be checked by some means. We are, 

 nevertheless, seldom able with certainty to tell in any given 



