indirect reasons we may feel sure, that in the southern hemisphere the
Macrauchenia, also, lived long subsequently to the ice-transporting
boulder-period. Did man, after his first inroad into South America,
destroy, as has been suggested, the unwieldy Megatherium and the other
Edentata? We must at least look to some other cause for the
destruction of the little tucutuco at Bahia Blanca, and of the many
fossil mice and other small quadrupeds in Brazil. No one will imagine
that a drought, even far severer than those which cause such losses in
the provinces of La Plata, could destroy every individual of every
species from Southern Patagonia to Behring's Straits. What shall we
say of the extinction of the horse? Did those plains fail of pasture,
which have since been overrun by thousands and hundreds of thousands of
the descendants of the stock introduced by the Spaniards? Have the
subsequently introduced species consumed the food of the great
antecedent races? Can we believe that the Capybara has taken the food
of the Toxodon, the Guanaco of the Macrauchenia, the existing small
Edentata of their numerous gigantic prototypes? Certainly, no fact in
the long history of the world is so startling as the wide and repeated
exterminations of its inhabitants.
Nevertheless, if we consider the subject under another point of view,
it will appear less perplexing. We do not steadily bear in mind, how
profoundly ignorant we are of the conditions of existence of every
animal; nor do we always remember, that some check is constantly
preventing the too rapid increase of every organized being left in a
state of nature. The supply of food, on an average, remains constant,
yet the tendency in every animal to increase by propagation is
geometrical; and its surprising effects have nowhere been more
astonishingly shown, than in the case of the European animals run wild
during the last few centuries in America. Every animal in a state of
nature regularly breeds; yet in a species long established, any _great_
increase in numbers is obviously impossible, and must be checked by
some means. We are, nevertheless, seldom able with certainty to tell in
any given species, at what period of life, or at what period of the
year, or whether only at long intervals, the check falls; or, again,
what is the precise nature of the check. Hence probably it is, that we
feel so little surprise at one, of two species closely allied in
habits, being rare and the other abundant in the same district; or,
again, that one should be abundant in one district, and another,
filling the same place in the economy of nature, should be abundant in
164