io6 THE EVOLUTION OF MAN 



of such organisms these tropistic tendencies are ordinarily 

 responded to no matter how untoward the consequences. The 

 moth seeking the flame is a common exemplar of this kind of 

 reaction. And the biologically undesirable consequences are 

 plainly exhibited in the demise of the. performer. 



The group of reactions called reflex exhibit a similar organic 

 invariability which, while generally beneficial, is not infre- 

 quently disadvantageous. In the human being sneezing, cough- 

 ing, weeping, are familiar examples of actions of this character. 

 They are largely outside the range of voluntary control and the 

 more extreme forms, such as those of digestion, are completely 

 independent of such control. 



Instincts shade off into reflex and tropistic reactions by 

 gradations, which makes it difficult without being arbitrary to 

 draw any hard and fast lines; but they are in the higher organ- 

 isms much less rigid and fixed than the tropisms and reflexes 

 and they are more complicated than the latter because they 

 involve a series of muscular movements, instead of the single 

 movement to which we apply the term reflex. 



All this group of activities, however, have in common their 

 hereditary and innate character. No one of the group is ever 

 in any proper sense learned or acquired. They are executed 

 either at birth, or at some later stage of the creature's de- 

 velopment, with a high degree of perfection and in the lower 

 animals they are generally quite rigid and devoid of any sug- 

 gestion of intelligent adaptation. They certainly represent 

 hereditary pathways through the nervous system over which 

 stimulations travel to the muscles and glands. In the higher 

 animals, the instincts are more or less plastic and susceptible 

 of modification, this modification in some cases going so far 

 as to result in the complete suppression of the instinct by 

 unfavorable environmental conditions. Most of the instincts 

 have a reasonably obvious biological utility, in that they con- 

 tribute to the maintenance of the life of the individual or the 



