experimental fact was so well known to prehistoric man that he
employed this method, as various savage tribes employ it to this
day, for the altogether practical purpose of making a fire; just
as he employed his practical knowledge of the mutability of
solids and liquids in smelting ores, in alloying copper with tin
to make bronze, and in casting this alloy in molds to make
various implements and weapons. Here, then, were the germs of an
elementary science of physics. Meanwhile such observations as
that of the solution of salt in water may be considered as giving
a first lesson in chemistry, but beyond such altogether
rudimentary conceptions chemical knowledge could not have
gone--unless, indeed, the practical observation of the effects of
fire be included; nor can this well be overlooked, since scarcely
another single line of practical observation had a more direct
influence in promoting the progress of man towards the heights of
civilization.

4. In the field of what we now speak of as biological knowledge,
primitive man had obviously the widest opportunity for practical
observation. We can hardly doubt that man attained, at an early
day, to that conception of identity and of difference which Plato
places at the head of his metaphysical system. We shall urge
presently that it is precisely such general ideas as these that
were man's earliest inductions from observation, and hence that
came to seem the most universal and "innate" ideas of his
mentality. It is quite inconceivable, for example, that even the
most rudimentary intelligence that could be called human could
fail to discriminate between living things and, let us say, the
rocks of the earth. The most primitive intelligence, then, must
have made a tacit classification of the natural objects about it