yptodons, and diprotodons, are uniparous. The actual presence,
therefore, of small species of animals in countries where larger species
of the same natural families formerly existed, is not the consequence of
degeneration--of any gradual diminution of the size--of such species,
but is the result of circumstances which may be illustrated by the fable
of 'the Oak and the Reed;' the smaller and feebler animals have bent and
accommodated themselves to changes to which the larger species have
succumbed."[68]

"We do not steadily bear in mind," remarks Mr Darwin, "how profoundly
ignorant we are of the condition of existence of every animal; nor do we
always remember that some check is constantly preventing the too rapid
increase of every organised 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 shewn, 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. If asked how this is, one immediately replies that it is
determined by some slight difference in climate, food, or the number of
enemies: yet how rarely, if ever, we can point out the precise cause and
manner of action of the check! We are, therefore, driven to the
conclusion that causes generally quite inappreciable by us, determine
whether a given species shall be abundant or scanty in numbers.

"In the cases where we can trace the extinction of a species through
man, either wholly or in one limited district, we know that it becomes
rarer and rarer, and is then lost; it would be difficult to point o