CHAPTER XII 
THE POSSIBLE ROLE OF ALCOHOL AND DISEASE 
IN CAUSING HEREDITARY DEFECTS 
“There is probably no biological problem of greater interest and 
importance, and about which less is known, than that of the causation 
of germinal variations—whether of a progressive or retrogressive 
nature.” —Tredgold, Mental Deficiency. 
In attempting to estimate the factors of evolution, whether 
in man or in the lower forms of life, we must of necessity face the 
problem of the causes of variability. Important as this subject 
is for evolutionary theory as well as many practical problems 
in experimental breeding, it has received surprisingly little 
attention from students of biology. Darwin, who studied varia- 
tion most exhaustively, and who amassed a great wealth of facts 
concerning the variations of animals and plants, threw little light 
upon the problem beyond pointing out the probability that 
“variability of every kind is directly or indirectly caused by 
changed conditions of life.” Domestication, especially if long 
continued, appears to enhance variability. In common with 
Andrew Knight, Schleiden and others Darwin held that excess of 
food is one of the most potent factors by which variations may be 
induced. Much of the variability due to food, climate, etc., 
was attributed by him to the inheritance of the somatic effects 
of these agencies,—a conclusion with which most geneticists 
would not now agree. Outer agencies were held also to affect 
the reproductive cells, and thus to cause variations which tend 
to become strongly inherited. 
Germinal variations frequently occur in a haphazard manner. 
Generally no specific cause can be assigned for their appearance. 
When a hairless dog, a navel orange, or a runnerless strawberry 
arises all we can say is that such events just happened. If con- 
genital variations arise as a response of the germ plasm to stimuli, 
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