EXTINCTION. 327 



Mastodon, Megatherium, Toxodon, and other extinct mon- 

 sters, which all co-existed with still living shells at a very 

 late geological period, I was tilled with astonishment ; for, 

 seeing that the horse, since its introduction by the Spaniards 

 into South America, has run wild over the whole country 

 and has increased in numbers at an unparalleled rate, I asked 

 myself what could so recently have exterminated the former 

 horse under conditions of life apparently so favorable. But 

 my astonishment was groundless. Professor Owen soon 

 perceived that the tooth, though so like that of the existing 

 horse, belonged to an extinct species. Had this horse been 

 still living, but in some degree rare, no naturalist would have 

 felt the least surprise at its rarity ; for rarity is the attribute 

 of a vast number of species of all classes, in all countries. 

 If we ask ourselves why this or that species is rare, we 

 answer that something is unfavorable in its conditions of 

 life ; but what that something is, we can hardly ever tell. 

 On the supposition of the fossil horse still existing as a rare 

 species, we might have felt certain, from the analogy of all 

 other mammals, even of the slow-breeding elephant, and 

 from the history of the naturalization of the domestic horse 

 in South America, that under more favorable conditions it 

 would in a very few years have stocked the whole continent. 

 But we could not have told what the unfavorable conditions 

 were which checked its increase, whether some one or several 

 contingencies, and at what period of the horse's life, and in 

 what degree, they severally acted. If the conditions had 

 gone on, however slowly, becoming less and less favorable, 

 we assuredly should not have perceived the fact, yet the 

 fossil horse would certainly have become rarer and rarer, 

 and finally extinct — its place being seized on by some more 

 successful competitor. 



It is most difficult always to remember that the increase 

 of every creature is constantly being checked by unper- 

 ceived hostile agencies ; and that these same unperceived 

 agencies are amply sufficient to cause rarity, and finally 

 extinction. So little is this subject understood, that I have 

 heard surprise repeatedly expressed at such great monsters 

 as the Mastodon and the more ancient Dinosaurians having 

 become extinct ; as if mere bodily strength gave victory in 

 the battle of life. Mere size, on the contrary, would in 

 some cases determine, as has been remarked by Owen, 

 quicker extermination, from the greater amount of requi- 

 site food. Before man inhabited India or Africa, some cause 



