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 Mendelian principles. On the contrary 
they illustrate to great advantage the previously 
given interpretation of all hereditary characters— 
namely, that every character is the realized 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 Mendelian 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 
