again. The fishes exchanged their ganoid
breast-plate for scales. Long before that, the insects had appeared,
also disencumbered of the breast-plate that had protected their
ancestors. Both supplemented the insufficiency of their protective
covering by an agility that enabled them to escape their enemies, and
also to assume the offensive, to choose the place and the moment of
encounter. We see a progress of the same kind in the evolution of human
armaments. The first impulse is to seek shelter; the second, which is
the better, is to become as supple as possible for flight and above all
for attack--attack being the most effective means of defense. So the
heavy hoplite was supplanted by the legionary; the knight, clad in
armor, had to give place to the light free-moving infantryman; and in a
general way, in the evolution of life, just as in the evolution of human
societies and of individual destinies, the greatest successes have been
for those who have accepted the heaviest risks.

Evidently, then, it was to the animal's interest to make itself more
mobile. As we said when speaking of adaptation in general, any
transformation of a species can be explained by its own particular
interest. This will give the immediate cause of the variation, but often
only the most superficial cause. The profound cause is the impulse which
thrust life into the world, which made it divide into vegetables and
animals, which shunted the animal on to suppleness of form, and which,
at a certain moment, in the animal kingdom threatened with torpor,
secured that, on some points at least, it should rouse itself up and
move forward.

On the two paths along which the vertebrates and arthropods have
separately evolved, development (apart from retrogressions connected
with parasitism or any other cause) has consisted above all in the
progress of the sensori-motor nervous system. Mobility and suppleness
were sought for, and also--through many experimental attempts, and not
without a tendency to excess of substance and brute force at the
start--variety of movements. But this quest itself took place in
divergent directions. A glance at the nervous system of the arthropods
and that of the vertebrates shows us the difference. In the arthropods,
the body is formed of a series more or less long of rings set together;
motor activity is thus distributed amongst a varying--sometimes a
considerable--number of appendages, each of which has its special
function. I