46 TYPES OF MENDELIAN HEREDITY 



duces an eye decidedly darker than normal. Such 

 cases of interaction of factors, in which the effect of 

 one factor is altered by the action of another factor, 

 are very numerous. 



IV. Conclusion 



It would have been indeed strange if Mendelian 

 factor-differences had not been found that require 

 special conditions — environmental, developmental, 

 or factorial — in order to produce a given effect, or 

 any effect at all. For Mendelian factors may cause 

 or influence all sorts of characters — that is, any or all 

 kinds of developmental or physiological reactions; 

 and many of these reactions are known to be affected 

 by age, temperature, region of the body, and so forth. 

 The facts given above are in no possible sense sub- 

 versive to MendeHan principles. On the contrary 

 they illustrate to great advantage the previously 

 given interpretation of all hereditary characters — 

 namely, that every character is the reahzed result of 

 the reaction of hereditary factors with each other 

 and with their environment. Failure to understand 

 this viewpoint has led to some futile criticism by the 

 opponents of the modern MendeHan interpretation 

 in terms of unit factors. This criticism is as pointless 

 as it would be to criticize the atomic theory on the 

 ground that oxygen does not, under all conditions, 

 and in all its compounds, give rise to substances with 

 the same properties. 



The validity of the unit factor conception rests 



