362 HEREDITY IN MAN 



nervous system itself. Though on this account temperament or 

 nervous tone is peculiarly susceptible to environmental influences, 

 a certain condition of nervous tone is innately given. We must 

 suppose the actual structure of the nervous system to be largely 

 inherited, and we must suppose that the manner in which the 

 bodily organs function is largely due to their innate organization. 

 We have thus to think of a certain temperament as always given 

 in the germinal constitution. An extreme example is an innate 

 defect of the thyroid gland which can produce any degree of mental 

 apathy. Again, such characters as excitability, rapidity of 

 response, and differences in respect of fatiguability and recupera- 

 tion are largely inherited. 



Habit is best considered under this heading. The essential fact 

 about habit is that, if the nervous system is stimulated in a certain 

 fashion, so that a stimulus passes along certain paths, the next 

 stimulus of the same kind will produce an impulse which passes 

 more easily along those paths. Thus in time the same result, 

 whatever it may be, is achieved by the application of a weaker 

 stimulus. This fact, which is of great importance, suggests an 

 actual modification of structure an actual fashioning of a path. 

 Whether this is so or not, we do not know. What is noticeable 

 here is that the ease with which habits are formed undoubtedly 

 differs from man to man, and we have again to postulate pre- 

 dispositions in the germinal constitution which tend towards 

 a definite degree of development of the power of the formation 

 of habits. 



5. The greatest^ difficulties arise when dealing with the purely 

 mental characters. There is a general agreement among biologists 

 that mental characters are inherited just as physical characters 

 are inherited. As we have pointed out, there is evidence to this 

 effect derived from studies on biometric lines. The difficulties 

 arise when we attempt to give more precision to the statement 

 that mental characters are inherited. One method of approaching 

 this problem is to analyse mental behaviour, and to ascertain 

 if possible what faculties there are which cannot be explained 

 in terms of other faculties. Such faculties are the least that we 

 must suppose to be given in the germinal constitution. It does 

 not, p_f_jeourse, follow that irreducible characters are Mendeliah 

 characters. In all probability they are due to the presence of 

 many Mendelian factors. This analysis is a matter of great 



