194 SECTIONAL ADDRESSES 
investigate its genetic basis, for it is possible that feeble-mindedness may 
be due to one or more of a large number of genetic factors ; there may be 
different forms of feeble-mindedness which are not distinguishable by 
means of intelligence-ratios. 
In investigations into the inheritance of intellect much reliance has been 
placed on rough-and-ready estimates based largely on social and professional 
success. In so far as such estimates are sound, these inquiries show that 
there is a correlation between the intelligence of parent and that of child, 
that bright parents have a higher proportion of bright children and that 
defective parents have a bigger proportion of defective children than do 
normal parents, but they have also shown that normal, even brilliant 
parents sometimes have defective children, that defective parents some- 
times have normal children, and they suggest that the mental deficiency 
of children of either bright or dull parents may be due either to external 
causes or to defective inheritance. The main facts have probably been 
made out, but the details are lacking, and will not be available until exact 
measurements have been made of the mental traits of parents and their 
children under conditions in which social opportunities and encourage- 
ments are equal for all. 
The theories of genetic inheritance which have proved so fruitful in 
the investigation of the physical characters of plants and the lower animals 
have been shown to apply also to human anatomical and physiological 
characters, such as the colour of the skin, stature, and susceptibility to 
disease, and it is probable that they apply also to mental characters ; if 
they do, then it is important that the characters should be distinguished 
and the manner of their inheritance traced out. The difficulties are great 
and for the most part obvious ; one is the difficulty of controlling environ- 
mental factors (the most humane method of overcoming this difficulty is 
to improve the conditions of life so as to give all a chance); another 
difficulty is that of finding really satisfactory tests for adults ; but perhaps 
the greatest of all is that of isolating and defining simple mental characters. 
Fortunately the last of these is a difficulty which we can hope soon to 
overcome, for the search for unitary mental traits has been proceeding 
vigorously, and there is now some prospect of diagnosing and measuring 
them, and so putting the study of genetic basis of mental traits on a sound 
footing. This will demand the co-operation on a big scale, not only of 
psychologists, but also of biologists, statisticians, teachers, medical men, 
and others, in which respect the study of mental inheritance resembles 
that of most other social problems. 
