Just as spontaneous variation is constantly pushing the animal or the plant to push out, as a vine its tendrils, in all directions, while natural conditions are as constantly exercising over it a sort of unconscious pruning power, so imagination is ever at work urging man's mind out and on, while the sentiment of the community, commonly called common sense, which simply means the point already reached by the average, is as steadily tending to keep it at its own level.

The environment helps, in the one case as in the other, to the shaping of the development.

Purely physical in the first, it is both physical and psychical in the second, the two reacting on each other.

But in either case it is only a constraining condition, not the divine impulse itself.

Precisely, then, as in the organism, this subtle spirit checked in one direction finds a way to advance in another, and produces in consequence among an originally similar set of bodies a gradual separation into species which grow wider with time, so in brain evolution a like force for like reasons tends inevitably to an ever-increasing individualization.
Now what evidence have we that this analogy holds?
Let us look at the facts, first as they present themselves subjectively.
The instinct of self-preservation, that guardian angel so persistent to appear when needed, owes its summons to another instinct no less strong, which we may call the instinct of individuality; for with the same innate tenacity with which we severally cling to life do we hold to the idea of our own identity.