e of certain groups of sensations or images (visual, tactile,
motor) that may be decisive in determining the direction of the
inventor.

(II) Mechanical invention grows by successive stratifications and
additions, as in the sciences, but more completely. It is a fine
verification of the "subsidiary law of growing complexity" previously
discussed.[125] If we measure the distance traversed since the distant
ages when man was naked and unarmed before nature to the present time of
the reign of machinery, we are astonished at the amount of imagination
produced and expended, often uselessly lavished, and we ask ourselves
how such a work could have been misunderstood or so lightly appreciated.
It does not pertain to our subject to make even a summary table of this
long development. The reader can consult the special works which,
unfortunately, are most often fragmentary and lack a general view. So we
should feel grateful to a historian of the useful arts, L. Bourdeau,
for having attempted to separate out the philosophy of the subject, and
for having fastened it down in the following formulas:[126]

(a) The exploitation of the powers of nature is made according to their
degree of power.

(b) The extension of working instruments has followed a logical
evolution in the direction of growing complexity and perfection.

Man, according to the observations of M. Bourdeau, has applied his
creative activity to natural forces and has set them to work according
to a regular order, viz.:

(1) Human forces, the only ones available during the "state of nature"
and the savage state. Before all else, man created weapons: the most
circumscribed primitive races have invented engines for attack and
defense--of wood, bone, stone, as they were able. Then the weapon became
a tool by special adaptation:--the battle-club serves as a lever, the
tomahawk as a hammer, the flint ax as a hatchet, etc. In this manner
there is gradually formed an arsenal of instruments. "Inferior to most
animals as regards certain work that would have to be done with the aid
of our organic resources alone, we are superior to all as soon as we set
our tools at work. If the rodents with their sharp teeth cut wood better
than we can, we do it still better with the ax, the chisel, the saw.
Some birds, with the help of a strong beak, by repeated blows,
penetrate the trunk of a tree: but the auger, the gimlet, the wimble do
the same work better and more quickly. The knife is