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.
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