geographical and geological changes we have been considering are probably
those which have been most effective in bringing about the great features
of the distribution of animals, as well as the larger movements in the
development of organised beings; but it is to the alternations of warm and
cold, or of uniform and excessive climates--of almost perpetual spring in
arctic as well as in temperate lands, with occasional phases of cold
culminating at remote intervals in glacial epochs,--that we must impute
some of the more remarkable changes both in the specific characters and in
the distribution of organisms.[47] {119} Although the geological evidence
is opposed to the belief in early glacial epochs except at very remote and
distant intervals, there is nothing which contradicts the occurrence of
repeated changes of climate, which, though too small in amount to produce
any well-marked physical or organic change, would yet be amply sufficient
to keep the organic world in a constant state of movement, and which, by
subjecting the whole flora and fauna of a country at comparatively short
intervals to decided changes of physical conditions, would supply that
stimulus and motive power which, as we have seen, is all that is necessary
to keep the processes of "natural selection" in constant operation.

The frequent recurrence of periods of high and of low excentricity must
certainly have produced changes of climate of considerable importance to
the life of animals and plants. During periods of high excentricity with
summer in _perihelion_, that season would be certainly very much hotter,
while the winters would be longer and colder than at present; and although
geographical conditions might prevent any permanent increase of snow and
ice even in the extreme north, yet we cannot doubt that the whole northern
hemisphere would then have a very different climate than when the changing
phase of precession brought a very cool summer and a very mild winter--a
perpetual spring, in fact. Now, such a change of climate would certainly be
calculated to bring about a considerable change of _species_, both by
modification and migration, without any such decided change of _type_
either in the vegetation or the animals that we could say from their fossil
remains that any change of climate had taken place. Let us suppose, for
instance, that the climate of England and that of Canada were to be
mutually exchanged, and that the change took five or six