Chap. X. 
EXTINCTION. 
319 
lessor 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 
been exterminated, either locally or wholly, through 
