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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