OBJECTIONS TO THE THEOET OF DESCENT. 339 



rare species, we might have felt certain, from the analogy 

 of all other mammals, even of the slow-hreeding elephant, 

 and from the history of the naturalization of the domes- 

 tic horse in South America, that under more favorahle 

 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 sever- 

 ally acted. If the conditions had gone on, however slow- 

 ly, becoming less and less favorable, we assuredly should 

 not have perceived the fact, yet the fossil horse would cer- 

 tainly have become rarer and rarer, and finally extinct ; 

 — its place being seized on by some more successful com- 

 petitor. 



It is most difficult always to remember that the in- 

 crease of every creature is constantly being checked by 

 unperceived hostile agencies ; and that these same unper- 

 ceived 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 requisite food. Before man inhab- 

 ited India or Africa, some cause must have checked the 

 continued increase of the existing elephant. A highly 

 capable judge. Dr. Falconer, believes that it is chiefly in- 

 sects which, from incessantly harassing and weakening 

 the elephant in India, check its increase ; and this was 

 Brace's conclusion with respfect to the African elephant 

 in Abyssinia. It is certain that insects and blood-suck- 



