208 MENDELISM chap. 



The analysis of mental characters will no doubt 

 be very difficult, and possibly the best line of attack 

 is to search for cases where they are associated with 

 some physical feature such as pigmentation. If an 

 association of this kind be found, and the pigmenta- 

 tion factors be determined, it is evident that we 

 should thereby obtain an insight into the nature of 

 the units upon which mental conditions depend. 

 Nor must it be forgotten that mental qualities, such 

 as quickness, generosity, instability, etc. — qualities 

 which we are accustomed to regard as convenient 

 units in classifying the different minds with which 

 we are daily brought into contact — are not neces- 

 sarily qualities that correspond to heritable units. 

 Effective mental ability is largely a matter of tem- 

 perament, and this in turn is quite possibly dependent 

 upon the various secretions produced by the different 

 tissues of the body. Similar nervous systems associ- 

 ated with different livers might conceivably result in 

 individuals upon whose mental ability the world 

 would pass a very different judgment. Indeed, it 

 is not at all impossible that a* particular form of 

 mental ability may depend for its manifestation, not 

 so much upon an essential difference in the structure 

 of the nervous system, as upon the production by 

 another tissue of some specific poison which causes 

 the nervous system to react in a definite way. We 

 have mentioned these possibilities merely to indicate 

 how complex the problem may turn out to be. 

 Though there is no doubt that mental ability is 

 inherited, what it is that is transmitted, whether 

 factors involving the quality and structure of the 

 nervous system itself, or factors involving the pro- 



