of limitless extent. By this it is not meant to imply that he had
a distinct conception of infinity, but, for that matter, it
cannot be said that any one to-day has a conception of infinity
that could be called definite. But, reasoning from experience and
the reports of travellers, there was nothing to suggest to early
man the limit of the earth. He did, indeed, find in his
wanderings, that changed climatic conditions barred him from
farther progress; but beyond the farthest reaches of his
migrations, the seemingly flat land-surfaces and water-surfaces
stretched away unbroken and, to all appearances, without end. It
would require a reach of the philosophical imagination to
conceive a limit to the earth, and while such imaginings may have
been current in the prehistoric period, we can have no proof of
them, and we may well postpone consideration of man's early
dreamings as to the shape of the earth until we enter the
historical epoch where we stand on firm ground.

2. Primitive man must, from a very early period, have observed
that the sun gives heat and light, and that the moon and stars
seem to give light only and no heat. It required but a slight
extension of this observation to note that the changing phases of
the seasons were associated with the seeming approach and
recession of the sun. This observation, however, could not have
been made until man had migrated from the tropical regions, and
had reached a stage of mechanical development enabling him to
live in subtropical or temperate zones. Even then it is
conceivable that a long period must have elapsed before a direct
causal relation was felt to exist between the shifting of the sun
and the shifting of the seasons; because, as every one knows, the
periods of greatest heat in summer and greatest cold in winter
usually come some weeks after the time of the solstices. Yet, the
fact that these extremes of temperature are associated in some
way with the change of the sun's place in the heavens must, in
time, have impressed itself upon even a rudimentary intelligence.
It is hardly necessary to add that this is not meant to imply any
definite knowledge of the real meaning of, the seeming
oscillations of the sun. We shall see that, even at a relatively
late period, the vaguest notions were still in vogue as to the
cause of the sun's changes of position.

That the sun, moon, and stars move across the heavens must
obviously have been among the earliest scientific observations.
It must not be inferred, however, that this observation implied a
necessary conception of the complete revolution of these bodies
about the earth. It is unnecessary to speculate here as to how
the primitive intelligence conceived the transfer of the sun from
the western to the eastern horizon, to be effected each night,
for we shall have occasion to examine some historical
speculations regarding this phenomenon. We may assume, however,
that the idea of the transfer of the heavenly bodies beneath the
earth (whatever the conception as to the form of that body) must
early have presented itself.

It required a relatively high development of the observing
faculties, yet a development which man must have attained ages
before the historical period, to note that the moon has a
secondary motion, which leads it to shift its relative position
in the heavens, as regards the stars; that the stars themselves,
on the other hand, keep a fixed relation as regards one another,
with the notable exception of two or three of the most brilliant
members of the galaxy, the latter being the bodies which came to
be known finally as planets, or wandering stars. The wandering
propensities of such brilliant bodies as Jupiter and Venus cannot
well have escaped detection. We may safely assume, however, that
these anomalous motions of the moon and planets found no
explanation that could be called scientific until a relatively
late period.

3. Turning from the heavens to the earth, and ignoring such
primitive observations as that of the distinction between land
and water, we may note that there was one great scientific law
which must have forced itself upon the attention of primitive
man. This is the law of universal terrestrial gravitation. The
word gravitation suggests the name of Newton, and it may excite
surprise to hear a knowledge of gravitation ascribed to men who
preceded that philosopher by, say, twenty-five or fifty thousand
years. Yet the slightest consideration of the facts will make it
clear that the great central law that all heavy bodies fall
directly towards the earth, cannot have escaped the attention of
the most primitive intelligence. The arboreal habits of our
primitive ancestors gave opportunities for constant observation
of the practicalities of this law. And, so soon as man had
developed the mental capacity to formulate ideas, one of the
earliest ideas must have been the conception, however vaguely
phrased in words, that all unsupported bodies fall towards the
earth. The same phenomenon being observed to operate on
water-surfaces, and no alteration being observed in its operation
in different portions of man's habitat, the most primitive
wanderer must have come to have full faith in the universal
action of the observed law of gravitation. Indeed, it is
inconceivable that he can have imagined a place on the earth
where this law does not operate. On the other hand, of course, he
never grasped the conception of the operation of this law beyond
the close proximity of the earth. To extend the reach of
gravitation out to the moon and to the stars, including within
its compass every particle of matter in the universe, was the
work of Newton, as we shall see in due course. Meantime we shall
better understand that work if we recall that the mere local fact
of terrestrial gravitation has been the familiar knowledge of all
generations of men. It may further help to connect us in sympathy


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