nsists, as we know, in
a change from a state of simple homogeneity to one of complex
heterogeneity. The process is apparently the same in a nebula or a
brachiopod, although much more intricate in the latter. The immediate
force which works this change, the life principle of things, is, in the
case of organic beings, a subtle something which we call spontaneous
variation. What this mysterious impulse may be is beyond our present
powers of recognition. As yet, the ultimates of all things lie hidden
in the womb of the vast unknown. But just as in the case of a man we can
tell what organs are vital, though we are ignorant what the vital spark
may be, so in our great cosmical laws we can say in what their power
resides, though we know not really what they are. Whether mind be but a
sublimated form of matter, or, what amounts to the same thing, matter
a menial kind of mind, or whether, which seems less likely, it be a
something incomparable with substance, of one thing we are sure, the
same laws of heredity govern both. In each a like chain of continuity
leads from the present to the dim past, a connecting clue which we can
follow backward in imagination. Now what spontaneous variation is to the
material organism, imagination, apparently, is to the mental one. 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 th