EVOLUTION OF NERVOUS SYSTEM 95 



system after all is only one of our numerous systems of organs 

 and lies embedded with the rest as part and parcel of our 

 bodies. Notwithstanding its unique character, the nervous 

 system is profoundly influenced by its organic environment and 

 reflects in much that comes from it the characteristics of the 

 body in which it lies. This is nowhere more clearly seen than 

 in the effects which hormones have upon it. Hormones, as you 

 already know, are substances produced in the bodies of the 

 higher animals by the ductless glands, or endocrine organs as 

 they are now called, and circulated by the blood. They are 

 powerful activators for many bodily reactions and are of first 

 importance in many organic activities. Certain hormones have 

 a profound effect upon the nervous system and consequently 

 upon personality. In this respect they are like the supposed 

 humors of the physicians of antiquity. These humors were 

 believed to be four in number, blood, phlegm, yellow bile, and 

 black bile, and an excess of any one of these gave a corre- 

 sponding temperament, sanguinary, phlegmatic, choleric, or 

 melancholic as the case might be. This idea of the humors is 

 really a forerunner of the well-established theory of the hor- 

 mones, which, however, not only has to do with temperament 

 but with many other functions of the body. That materials 

 introduced into the circulation have a profound influence on 

 our moods and other mental states is well known and is the 

 basis of the use and abuse of many substances, such as nar- 

 cotics, alcohol, coffee, tea, tobacco, and so forth. It is, how- 

 ever, only within the last few years that the immense impor- 

 tance of hormones in relation to the nervous system has begun 

 to be seen. The hormones concerned are those that have to 

 do with the sexual traits. 



The great majority of animals are either male or female 

 and when we seek for a definition of these states we are driven 

 in the end to make it turn upon the kind of reproductive cells 

 that the individual produces. Males produce sperm cells; 



