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 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 naturalisation 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, 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
living 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 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 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 exterminated, either locally
or wholly, through man's a