o many conflicting agents can be definitely
settled. The causes in simultaneous action are so numerous, that they
admit of an almost infinite number of combinations; and it is necessary
that all these should have occurred once before the total amount of
change, capable of flowing from any new disturbing force, can be
estimated.

Thus, for example, suppose that once in two centuries a frost of unusual
intensity, or a volcanic eruption of great violence accompanied by
floods from the melting of glaciers, should occur in Iceland; or an
epidemic disease, fatal to the larger number of individuals of some one
species, and not affecting others,--these, and a variety of other
contingencies, all of which may occur at once, or at periods separated
by different intervals of time, ought to happen before it would be
possible for us to declare what ultimate alteration the presence of any
new comer, such as the bear before mentioned, might occasion in the
animal population of the isle.

Every new condition in the state of the organic or inorganic creation, a
new animal or plant, an additional snow-clad mountain, any permanent
change, however slight in comparison to the whole, gives rise to a new
order of things, and may make a material change in regard to some one or
more species. Yet a swarm of locusts, or a frost of extreme intensity,
or an epidemic disease, may pass away without any great apparent
derangement; no species may be lost, and all may soon recover their
former relative numbers, because the same scourges may have visited the
region again and again, at preceding periods. Every plant that was
incapable of resisting such a degree of cold, every animal which was
exposed to be entirely cut off by an epidemic or by famine caused by the
consumption of vegetation by the locusts, may have perished already, so
that the subsequent recurrence of similar catastrophes is attended only
by a temporary change.

_Changes caused by Man_

We are best acquainted with the mutations brought about by the progress
of human population, and the growth of plants and animals favored by
man. To these, therefore, we should in the first instance turn our
attention. If we conclude, from the concurrent testimony of history and
of the evidence yielded by geological data, that man is, comparatively
speaking, of very modern origin, we must at once perceive how great a
revolution in the state of the animate world the increase of the human
race, conside