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unknown date of the fire revolution, we know that
56
THE FOUR REVOLUTIONS
within less than the past twenty thousand years there
have already been three revolutions. If this can be re-
garded as a precedent it suggests that there should be a
revolution at least every ten thousand years; that is to say
more than a hundred of them in the span of a million
years. I confess to very considerable doubt as to the
likelihood that there are so many revolutions in store
for our descendants, but at any rate it is hardly profitable
to speculate further on the subject.
57
MATERIAL CONDITIONS
HE future of the human race of course must
T
depend on the nature of the inorganic world in
which it lives, so that it is well to begin by reviewing
this. In the first place all astronomical and geological
evidence indicates that the climate of the earth has been
roughly constant for more than a thousand million
years, and there is every reason to think it will continue
so for many million years to come. There is always of
course the chance that there may be a dark star moving
through space towards the solar system so as to collide
with it. The collision need not be very severe to end the
history of the human race, for a perturbation of the
earth's orbit, which might from the astronomical point
of view be counted as quite small, would be sufficient
to change the climate enough to destroy all life.
We obviously cannot know whether there is a dark
star approaching us, because it would be invisible until it
was quite near, but we can say that it is extremely im-
probable. First, if there were many such stars, one of
them would probably already have hit the solar system
during the era of two thousand million years for which
the earth has existed. Secondly, in the intensive study of
the heavens by astronomers, collisions would have been
observed between other stars, and though new stars,
58
MATERIAL CONDITIONS
novae, are found rather frequently, their character does
not suggest that they were caused by collisions of this
kind. There is also another class of new stars, the super-
novae, only rather recendy recognized; the last one that
occurred in the galaxy was Tycho's star, which hap-
pened in 1572 for some time it was so bright as to be
visible in daylight. It is still very doubtful what makes a
supernova; it might be a stage in the life of every star,
but their rarity makes this unlikely, and as the sun is by
all standards a very normal, astrophysically uninteresting
star, we can be fairly sure that it will not blow up in this
way. The general conclusion of the astronomical evid-
ence is that it is very unlikely indeed that there should
be a catastrophic end to the earth in a million years, or
any substantial change in its condition.
Though the earth's climate has been roughly con-
stant for so long, there have been minor fluctuations in it.
Thus in England we are only now emerging from an ice
age. This is the last of four recent periods of glaciation
in the northern hemisphere, and there were three inter-
missions between these periods when the climate was
even warmer than it now is for quite a long time. We
cannot therefore be sure that there are no further ice ages
coming to us. All that can be said is that though there
have undoubtedly been other ice ages in the more distant
past, they are geologically speaking rather rare events.
Also theorists claim to have given an explanation on
astronomical grounds for the recent four ages but then
if there had been five, might they not have discovered a
different but equally cogent reason for there having been
five? So we cannot be quite sure that there may not be
59
THE NEXT MILLION YEARS
more of them to come within a few tens of thousands of
years. However, these things are trivial, for as first Scan-
dinavia, then Scotland and then England became unin-
habitable, so the climate further south would improve;
rain would fall in the Sahara, agriculture would flourish
there and a general shift of populations southwards
would leave things much as they are.
In this connection the direct influence that civilized
man has had on geography may be noted. Less than ten
thousand years ago England was connected with Europe
over what is now the North Sea. This region was
gradually drowned and, but for the direct action of
man, most of Holland, and the English fen country
would by now also have been drowned and indis-|
tinguishable from the North Sea. But these are com-
paratively minor matters, for the evidence of the past
shows that the sea level has altered up and down quite
considerably on account of the varying amount of ice
locked up at the poles, and it is evident that as trivial a
change as fifty feet in the level of the sea would entirely
defeat man's efforts to preserve the low lying regions, or
conversely with a rather larger change in the other
direction would make it impossible to preserve Britain
as an island. Man's direct influence on geography is
really quite negligible. On the other hand his indirect
influence on geography has been more considerable,
since he has made very perceptible changes in climate by
the felling of forests. This felling tends to remove the
spongy cover of the ground which acts as a reservoir for
water, and it leads to a consequent erosion of his fields.
All this is now very much on the public conscience, and [ Pobierz całość w formacie PDF ]