acting upon fortuitous variations, be
the sole guiding process concerned in progress? Must there not be
some combining power to produce the higher individuals which are
prerequisites to the working of natural selection?

We are considering the efficiency of natural selection in enhancing
useful variations through a series of generations. Let us return to
the distinction between productiveness and prospectiveness of social
capital. Applied to variations productiveness means immediate
advantage, prospectiveness the greater future and permanent returns.
Now all persisting variations must, in animals below man, apparently
be somewhat productive, else they would not continue, much less
increase. Now the immediate return from prospective variations is
often smaller than from productive. It looks at first as if
productive variations would always be preserved by natural
selection, and that prospective variations would not long advance.
Yet in the muscular system variations valuable largely for their
future value are neither few nor unimportant. How can the brain in
its infancy develop until it gains supremacy over muscle, or muscle
have done the same with digestion? Now a partial explanation of this
is to be found in the correlation of organs. This is therefore a
factor of vast importance in progress through evolution.

Progress in any one line demands correlated changes in many organs.
Thus in the advance of annelids to insects the muscular system
increases in relative bulk, and absolutely in complexity. But a
change or increase in the muscle must be accompanied by
corresponding changes in the motor-nerve fibrils; and these again
would be useless unless accompanied by increased complexity and more
or less readjustment of the cells and fibrils of the nerve-centres.
And all these additions to, and readjustments of, the nerve-centres
must take place without any disturbance of the other necessary
adjustments already attained. This is no simple problem.

We will here neglect the fact that many other changes are going on
simultaneously. Legs are being formed or moulded into jaws, the
anterior segments are fusing into a head, and their ganglia into a
brain; an external skeleton is developing. Furthermore the increase
of the muscular and nervous systems must be accompanied by increased
powers of digestion, respiration, and excretion. Practically the
whole body is being recast. We insist only on the necessity of
simultaneous and p