370 ORIGIN OF SPECIES 



elephant, and from the history of the naturalisation of 

 the domestic horse in South America, that under more 

 favourable conditions it would in a very few years have 

 stocked the whole continent. But we could not have told 

 what the unfavourable conditions were which checked its in- 

 crease, 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 favourable, 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 com- 

 petitor. 



It is most difficult always to remember that the increase 

 of every creature is constantly being checked by unperceived 

 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 sur- 

 prise repeatedly expressed at such great monsters as the 

 Mastodon and the more ancient Dinosaurians having be- 

 come 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 continued increase of the existing ele- 

 phant. 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 ele- 

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

 sucking bats determine the existence of the larger natural- 

 ized quadrupeds in several parts of S. America. 

 ^ We see in many cases in the more recent tertiary forma- 

 tions, that rarity precedes extinction ; and we know that this 

 has been the progress of events with those animals which 

 have been exterminated, either locally or wholly, through 

 man's agency. I may repeat what I published in 1845, 

 namely, that to admit that species generally become rare 

 before they become extinct — to feel no surprise at the rarity 



