Chap. XL] EXTINCTION. 295 



certain, from the analogy of all other mammals, even of the slow- 

 breeding elephant, and from the history of the naturalisation of the 

 domestic horse in South America, that under more favourable con- 

 ditions it would in a very few years have stocked the whole 

 continent. But we could not have told what the unfavourable con- 

 ditions 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, how- 

 ever 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 competitor. 



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 under- 

 stood, that 1 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 continued 

 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 naturalised quadrupeds ia 

 several parts of S. America. 



We see in many cases in the more recent tertiary formations, 

 that rarity precedes extinction ; and we know that this has been 

 the progress of events with those animals which have been exter- 

 minated, 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 sur- 

 prise at the rarity of a species, and yet to marvel greatly when the 

 species ceases to exist, is much the same as to admit that sickness 

 in the individual is the forerunner of death — to feel no surprise 

 at sickness, but, when the sick man dies, to wonder and to suspect 

 that he died by some deed of violence. 



The theory of natural selection is grounded on the belief that 

 each new variety and ultimately each new species, Is produced and 



