INHERITANCE OF MENTAL DEFECT 67 



It is possible that mental defect, like hereditary 

 ability at the other end of the scale, is no simple 

 quality ; but, although complex in its character, it is 

 probably a simpler aggregation of factors than tran- 

 scendent genius. Mental defect is easier to define, 

 easier to recognise, and therefore easier to trace. In 

 many cases the physical malformations are manifest 

 to every one ; the want of mind of the imbecile is 

 as obvious as the aberration of mind of the lunatic. 

 Border-line cases there must be ; but these, too, will 

 often slip into their correct classification in periods 

 of mental strain, of excitement, of ill-regulated life. 

 Whether we shall ever be able to separate out the 

 various strains and deal with each of them as a definite 

 Mendelian unit remains to be seen ; at present, at any 

 rate, the many pedigrees of degenerate stocks that have 

 accumulated help us to realize the interchangeableness of 

 some of the various manifestations and the direct influence 

 of heredity on their transmission from parent to offspring. 



Two such pedigrees are given below, to illustrate the 



descent of mental defect in its various forms through 



four generations. 



CASE I 



Father, = Mother, 



eccentric. 



insane. 



daughter, 

 mentally defective — unmarried. 



daughter, daughter, 



mentally defective. almost imbecile — 



I at present detained under 



I twins. I Industrial Schools Act. 

 daughter, daughter, 

 died at ! month. died at 6 months. 



