CHAPTER III 
THE INHERITANCE OF MENTAL DEFECTS AND 
DISEASE 
“Our human civilized stock is far more weakly through congenital 
imperfection than that of any other species of animals, whether wild or 
domestic.”’—Francis Galton, Inquiries into Human Faculty. 
THAT many forms of mental deficiency and disorder are capable 
of hereditary transmission, has long been recognized, but it is 
only recently that attempts have been made to discover the 
precise rules according to which such transmission takes place. 
Much, however, still remains obscure in regard to this important 
topic. The vast literature on the subject contained in works on 
medicine and pathology, in numerous medical journals and va- 
rious other publications consists mainly in the discussion of iso- 
jated cases of transmission, or the compilation of mass statistics 
from the records of institutions for the care of the mentally ab- 
normal. Institutional records being often gathered in a more or 
less perfunctory manner, and by many different persons, are apt 
to include numerous inaccuracies and are pretty sure to fall short 
of the desired degree of fullness. The relatives of mental defec- 
tives from motives of family pride frequently conceal the exist- 
ence of defects in other members of the family, and even when 
they honestly attempt to give all the information they possess 
they often fail to furnish data of any value. 
It is not surprising, therefore, to encounter wide differences of 
opinion among authorities concerning the extent to which various 
forms of defect depend upon a hereditary diathesis. Practically 
everyone whose opinion is of any value concedes to heredity a 
certain réle in the causation of neuropathic traits. A part of the 
difference of opinion doubtless depends upon the circumstance 
that the relative potency of hereditary and environmental factors 
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