INHERITANCE OF MENTAL DEFECTS AND DISEASE 61 
severely criticized, and justly so, by Pearson, Heron, Saleeby, 
and others. Granting that mental defect is transmitted as a 
single recessive unit character, the mating of a duplex normal 
with a defective, while producing normal children, nevertheless 
makes them carriers of the defect. Should two such carriers mate, 
one-fourth of their offspring would manifest the defect; should the 
carriers follow the ‘“‘eugenic rule” and mate with defectives, half 
of their offspring would be defective. Matings of normal and 
defective simply sow the seed for future trouble. Should the 
estimate of some of the workers of the Eugenics Record Office 
prove correct, namely, that over 30 per cent of the population is 
heterozygous for mental defect, the direct danger of such matings 
is very considerable. Certain defects are distributed widely 
enough as it is, without our advising marriages that would simply 
make the situation worse. Nothing could be more inconsistent 
with everything we know of heredity than the ill-considered 
advice that strength may mate with weakness. 
And besides we have very little assurance that the normal 
condition dominates mental defectiveness to the extent that is 
usually assumed. I have been continually surprised in reading 
papers on the Mendelian inheritance of mental defect to find how 
placidly and uncritically the assumption is made that normal 
mentality behaves as a typical dominant. It does not seem to 
occur to most of those who have treated the subject that the 
children of a mental defective are apt to be severely injured by 
the incompletely suppressed traits of that parent, however free 
from taint the ancestry of the other parent may have been. And 
this in spite of the fact that Mendelian literature is full of cases 
of incomplete and variable dominance! Surely from the facts 
at our disposal no one is justified in feeling very confident of 
the complete dominance of mental normality. The injury result- 
ing from the mating of mental soundness with mental weakness 
may be very direct, manifesting itself in the production of chil- 
dren mentally inferior or suffering from various neuropathic 
taints. It is not at all unlikely that many of them would actually 
be ranked as mental defectives or be caused by untoward circum- 
