Sensation 79 



involves the development of motor organs. 

 Structure not needed to sustain activity can 

 serve, by becoming passive, as a channel for 

 currents excited by external relations, and con- 

 vey to the mind indications of what takes place 

 on the outside. Sensation, under these condi- 

 tions, is the utilization of disused motor organs 

 for the purpose of receiving impressions. 1 

 Disused parts acquire a new use by forwarding 

 currents inward. Before this differentiation 

 survival is purely accidental; after it, the en- 

 vironment acts on organisms, and gives an 

 advantage to the best developed. The external 

 now becomes known, and the better move- 

 ments of the more developed organs assist in 

 adjustment or in the avoidance of evils. A 

 new epoch of development is entered, in which 

 consciousness is sustained by the continuous 

 inflow of impressions instead of the early de- 

 pendence on the spontaneous activity of the 

 germ cell. 



Let us picture this change on the organic 



1 See the writer's article on " Overnutrition and its Social 

 Consequences 11 in the Annals of the American Academy of 

 Political and Social Science, July, 1897. 



