Chap. X. EXTINCTION. 319 



how utterly groundless was my astonishment ! Pro- 

 fessor Owen soon perceived that the tooth, though so 

 like that of the existing horse, belonged to an extinct 

 species. Had this horse been still living, but in some 

 degree rare, no naturalist would have felt the least sur- 

 prise at its rarity ; 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 unfavourable in its conditions 

 of life ; but what that something is, we can hardly ever 

 tell. On the supposition of the fossil horse still exist- 

 ing 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 natural- 

 isation 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 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, becom- 

 ing less and less favourable, we assuredly should not 

 have perceived the fact, yet the fossil horse would cer- 

 tainly 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 living being is constantly being 

 checked by unperceived injurious agencies; and that 

 these same unperceived agencies are amply sufficient to 

 cause rarity, and finally extinction. 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 



