ken place,
at epochs of the most remote antiquity, in the climate and temperature
of our globe; changes difficult to reconcile with the operation of
secondary causes, such as a different distribution of sea and land, but
which would find an easy and natural explanation in a slow variation of
the supply of light and heat afforded by the sun himself."[276] "I can
not otherwise understand alterations of heat and cold so extensive as at
one period to have clothed high northern latitudes with a more than
tropical luxuriance of vegetation, and at another to have buried vast
tracts of Europe, now enjoying a genial climate, and smiling with
fertility, under a glacier crust of enormous thickness. Such changes
seem to point to causes more powerful than the mere local distribution
of land and water can well be supposed to have been. In the slow secular
variations of our supply of light and heat from the sun, _which, in the
immensity of time, may have gone to any extent, and succeeded each other
in any order, without violating the analogy of sidereal phenomena which
we know to have taken place_, we have a cause, not indeed established as
a fact, but readily admissible as something beyond a bare possibility,
fully adequate to the utmost requirements of geology. A change of half a
magnitude on the luster of our sun, regarded as a fixed star, spread
over successive geological epochs--now progressive, now receding, now
stationary--_is what no astronomer would now hesitate to admit as a
perfectly reasonable and not improbable supposition_."[277]

The most eminent astronomers are perfectly unanimous in their deductions
from these facts. They regard _variability as the general characteristic
of suns and stars, our own sun not exempted_. "We are led," says
Humboldt, "by analogy to infer, that as the fixed stars _universally_
have not merely an apparent, but a real motion of their own, so their
surfaces or luminous atmospheres are generally subject to those changes
(in their "light process") which recur, in the great majority, in
extremely long, and therefore unmeasured, and probably undeterminable
periods, or which, in a few, recur without being periodical, as it were,
by a sudden revolution, either for a longer or a shorter time." And he
asks, _Why should our sun differ from other suns?_

In reference to the extinction of suns, he says: "What we no longer see
is not necessarily annihilated. It is merely the transition of matter
into new