EVOLUTIONAL PSYCHOLOGY 77 



condition of the reproduction of mind-images there must be 

 " depth of impressions," and there must also exist a further 

 circumstance, known as " the force of association." " Most 

 of the events of life," so we are told, " are forgotten just because 

 they never recur in precisely the same form. The bulk of our 

 mental imagery answers to objects which we see again and again, 

 and events which repeatedly occur." " The more frequently 

 an impression is repeated, the more enduring will be the image. 

 Where the repetition of the actual impression is impossible, the 

 repeated reproduction of it serves less effectually to bring about 

 the same result." 



Again we meet with the important pre-requisite of " per- 

 manence " in the growth of mind. As before, it clearly emerges 

 that social interaction is the most effective means of building 

 up mind. We may once again conclude that nothing so much 

 as systematic biological co-operation could have produced the 

 right psychological foundation of the human mind. Repeti- 

 tions and mere frequency of impressions per se cannot be looked 

 upon as sufficient ; for they are not by themselves guarantees 

 of survival capacity. It is necessary that the respective activ- 

 ities be of a " right " kind, i.e,, sanctioned by the biological 

 use they serve. We shall see presently that not only regular- 

 isation and due frequency, but also due limitation of sense 

 impressions is necessary to produce desirable permanent effects, 

 and this precisely as though the conditions generally desirable 

 to achieve really " good " psychological results in the case of 

 man were also those required for the purposes of Symbiosis, with 

 its sine qua non of moderation. Obviously again, the hazards 

 and vicissitudes of the savage and predaceous life cannot supply 

 conditions the equals in general beneficence of those furnished 

 by the symbiotic life with the regularity, health and security 

 that it entails. 



The increase of brain power is recognised to be due to exercise, 

 and this, according to Professor Sully, implies two things : 



(1) All brain activity reacts on the particular structure engaged, 

 modifying it in some unknown way and bringing about a subsequent 



physiological disposition " to act in a similar manner. 



(2) In the second place we have to assume that different parts of the 

 brain which are exercised together acquire in some way a disposition to 

 conjoint action. 



