must always intervene and
prevent the actual accomplishment of such conversions. A faint image of
the certain doom of a species less fitted to struggle with some new
condition in a region which it previously inhabited, and where it has to
contend with a more vigorous species, is presented by the extirpation of
savage tribes of men by the advancing colony of some civilized nation.
In this case the contest is merely between two different _races_--two
varieties, moreover, of a species which exceeds all others in its
aptitude to accommodate its habits to the most extraordinary variations
of circumstances. Yet few future events are more certain than the speedy
extermination of the Indians of North America and the savages of New
Holland in the course of a few centuries, when these tribes will be
remembered only in poetry or history.

_Concluding remarks._--We often hear astonishment expressed at the
disappearance from the earth in times comparatively modern of many small
as well as large animals, the remains of which have been found in a
fossil state, under circumstances implying that neither any great
geographical revolution, nor the exterminating influence of man has
intervened to account for their extinction. But in all such cases we
should inquire whether we are sufficiently acquainted with the numerous
and complicated conditions on which the perpetuation of each species
depends, to entitle us to wonder if it should be suddenly cut off.

Mr. Darwin, when calling attention to the fact that the horse,
megatherium, megalonyx, and many contemporary Mammalia, had perished in
South America after that continent had acquired its present
configuration, and when, if we may judge by the Testacea, the climate
very nearly resembled the present, observes, "that in the living
creation one species is often extremely rare in a given region, while
another of the same genus and with closely allied habits is exceedingly
common. A zoologist familiar with such phenomena, if asked to explain
them, usually replies, that some slight difference in climate, food, or
the number of its enemies, must determine the relative strength of the
two species in question, although we may be unable to point out the
precise manner of the 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. Why, then, should we feel astonishmen