lie in other directions, and so to keep up the normal structure of the species.  The working out of the process is here somewhat difficult to follow; but it appears to me that as fast as the number of bodily and mental faculties increases, and as fast as the maintenance of life comes to depend less on the amount of any one, and more on the combined action of all, so fast does the production of specialties of character by natural selection alone become difficult.  Particularly does this seem to be so with a species so multitudinous in its powers as mankind, and above all does it seem to be so with such of the human powers as have but minor shares in aiding the struggle for life—­the aesthetic faculties for example.”—­Spencer, “Principles of Biology,” sec. 166.

Can thus natural selection, 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 parallel changes in muscles, nerves, and nerve-centres; though what is true of these is true, in greater or less degree, of all the other organs.