184 
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 
America, 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 
Toxodon, 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 
