300 EXTINCTION, Chap. X. 



iodon, Mcgaihcrium, Toxodon, and other extinct monsters, 

 ^vllich all coexisted with still liviiii? sliells at a very late geolo- 

 gical period, I Avas filled Avilh astonishment; for, seeing that 

 the horse, since its introduction by the Spaniards into South 

 America, has run wild over the Avholc country, and has in- 

 creased in numbers at an unparalleled rate, I asked myself 

 what could so recently have exterminated the former horse un- 

 der conditions of life apparently so favorable. But my aston- 

 ishment was groundless. Prof. Owea 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 Avould have felt the least sur- 

 prise 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-breed- 

 ing ele])hant, and from the history of the naturalization of the 

 domestic horse in South Amei'ica, that, xuider more favorable 

 conditions, it would, in a very few years, have stocked the 

 whole continent. 13ut wc could not have told what the un- 

 favorable conditions Avere which checked its increase, Avhether 

 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 w^ould certainly have become rarer and 

 rarer, and finally extinct — its place being seized on by some 

 more successful competitor. 



It is most diilicult alwa3's to renaembcr that the increase 

 of every creature is constantly being checked by imperceived 

 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 re- 

 peatedly expressed at such great monsters as the Mastodon 

 and the more ancient Dinosaurians having become extinct; as 

 if mere bodily strength ga\e 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 requisite food. Before man inhabited India or Africa, 

 some cause must have checked the continued increase of the 



