nd
mitigation of the climates otherwise produced, seems beyond question.
And it seems also beyond question that there must have been a consequent
rhythmical change in the distribution of organisms--a rhythmical change
to which we here wish to draw attention, as one cause of minor breaks in
the succession of fossil remains. Each species of plant and animal has
certain limits of heat and cold within which only it can exist; and
these limits in a great degree determine its geographical position. It
will not spread north of a certain latitude, because it cannot bear a
more northern winter, nor south of a certain latitude, because the
summer heat is too great; or else it is indirectly restrained from
spreading further by the effect of temperature on the humidity of the
air, or on the distribution of the organisms it lives upon. But now,
what will result from a slow alteration of climate, produced as above
described? Supposing the period we set out from is that in which the
contrast of seasons is least marked, it is manifest that during the
progress towards the period of most violent contrast, each species of
plant and animal will gradually change its limits of distribution--will
be driven back, here by the winter's increasing cold, and there by the
summer's increasing heat--will retire into those localities that are
still fit for it. Thus during 10,000 years, each species will ebb away
from certain regions it was inhabiting; and during the succeeding 10,000
years will flow back into those regions. From the strata there forming,
its remains will disappear; they will be absent from some of the
superposed strata; and will be found in strata higher up. But in what
shapes will they re-appear? Exposed during the 21,000 years of their
slow recession and their slow return, to changing conditions of life,
they are likely to have undergone modifications; and will probably
re-appear with slight differences of constitution and perhaps of
form--will be new varieties or perhaps new sub-species.

To this cause of minor breaks in the succession of organic forms--a
cause on which we have dwelt because it has not been taken into
account--we must add sundry others. Besides these periodically-recurring
changes of climate, there are the irregular ones produced by
redistributions of land and sea; and these, sometimes less, sometimes
greater, in degree, than the rhythmical changes, must, like them, cause
in each region emigrations and immigrations