GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 301 



for rarity is the attribute of a vast number of species of all classes, 

 in all countries. If we ask ourselves why this or that species is 

 rare, we answer that something is unfavorable in its conditions of 

 life; but what that something is, we can hardly ever tell. On the 

 supposition of the fossil horse still existing as a rare species, we 

 might have felt certain, from the analogy of all other mammals, 

 even of the slow-breeding elephant, and from the history of the 

 naturalization of the domestic horse in South America, that under 

 more favorable 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 severally acted. If the con- 

 ditions had gone on, however slowly, becoming less and less favor- 

 able, we assuredly should not have perceived the fact, yet the fos- 

 sil 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 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 con- 

 trary, would in some cases determine, as has been remarked by 

 Owen, quicker extermination, from the greater amount of requi- 

 site 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 weaking the ele- 

 phant in India, check its increase; and this was Bruce's conclu- 

 sion with respect to the African elephant in Abyssinia. It is cer- 

 tain that insects and bloodsucking 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 formations, 

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

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

 terminated, 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 



