104 THE ORIGIN OF SPECIES 



in a very few years have stocked the whole continent. 

 But we could not have told what the unfavorable condi- 

 tions 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 per- 

 ceived 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 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 inhabited 

 India or Africa, some cause must have checked the con- 

 tinued increase of the existing elephant. A highly 

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

 insects which, from incessantly harassing and weakening 

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

 Bruce's conclusion with respect to the African elephant 

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

 bats determine the existence of the larger naturalized 

 quadrupeds in several parts of South America. 



We see in many cases in the more recent tertiary 



