An Examination, Moreover, Of The Geology Of
La Plata And Patagonia, Leads To The Belief That All The
Features Of The Land Result From Slow And Gradual Changes.
It
appears from the character of the fossils in Europe, Asia,
Australia, and in North and South America, that those conditions
which favour the life of the _larger_ quadrupeds were
lately co-extensive with the world:
What those conditions
were, no one has yet even conjectured. It could hardly have
been a change of temperature, which at about the same time
destroyed the inhabitants of tropical, temperate, and arctic
latitudes on both sides of the globe. In North America we
positively know from Mr. Lyell, that the large quadrupeds
lived subsequently to that period, when boulders were
brought into latitudes at which icebergs now never arrive:
from conclusive but 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 a neighbouring
district, differing very little in its conditions.
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