CHAP. Til] EXTINCTION. 277 



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 surprise repeatedly ex- 

 pressed 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 Brace'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 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 surprise 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 

 maintained by having some advantage over those with which it 

 comes into competition; and the consequent extinction of the 

 less -favoured forms almost inevitably follows. It is the same 

 with our domestic productions ; when a new and slightly improved 

 variety has been raised, it at first supplants the less improved 

 varieties in the same neighbourhood ; when much improved it 

 is transported far and near, like our short-horn cattle, and take? 

 the place of other breeds in other countries. Thus the appearance 

 o new forms and the disappearance of old forms, both those 

 naturally and those artificially produced, are bound together. 

 In flourishing groups, the number of new specific forms which 

 have been produced within a given time has at some periods 

 probably been greater than the number of the old specific forms 

 which have been exterminated; but we know that species 



