254 BIOLOGICAL ASPECTS OF HUMAN PROBLEMS 



reaction of its protoplasm and mainly its nervous 

 protoplasm toward the environmental forces which 

 serially impinge on it. If the impinging forces of 

 environment are at present so numerous, so com- 

 plexly interwoven, that we imperfectly comprehend 

 them, what right have we to talk of their future 

 effects, in the presence of an undeniable tendency to 

 still further complication of forces? And correl- 

 atively, if we know so little of the basic forces of 

 protoplasm and the memories engraven there, how 

 shall we presume to predict the future properties 

 and reactions of this inexplicable maze of material? 

 Discouraging as is any truly scientific approach to 

 the prediction of human developmental tendency, it 

 does no harm to realize the nature of the difficulties, 

 even though such realization serves no other purpose 

 than that of preventing us from being too exacting 

 in our prophetic demands. 



I propose to consider here, in a brief way, the 

 various types of human nervous reaction with which 

 we are acquainted, in their relation to human develop- 

 ment. Empirical experience must be our main guide 

 in this endeavor and, strictly speaking, most of 

 what we know of biology may without injustice be 

 classed as empirical knowledge. As already pointed 

 out repeatedly, the fundamental nervous reactions 

 in man, as in lower animals, are only two in number: 

 the self -preservative and the sexual. While these 

 distinct reactions subserve quite distinct though 

 harmonious biological ends, the reactions are linked 



