458 H. J. MULLER 
Of course, when it is stated that these eye-color changes depend 
upon ‘qualitative,’ rather than ‘quantitative’ differences in the 
gene, reference is made here merely to the gene ‘in gross’ (if we 
may be pardoned such an expression); it is not denied that these 
qualitative changes themselves may perhaps consist in quanti- 
tative changes of some part of the gene (e.g., in the proportion 
of certain kinds of radicles or even molecules). But if we think 
of the changes as quantitative in this sense, then we must recog- 
nize that such a quantitative change of a portion of a gene, with 
the rest of it perhaps remaining constant, must be an entirely 
different thing from a quantitative change in the gene as a whole, 
with the neighboring genes and the rest of the chromosome 
remaining constant. For in the first case a marked effect is 
produced on development (manifested in a different eye color, 
for example); the new genic structure thus exerts a positive 
action different from that exerted by the original structure, an 
action which may interfere with the latter when both genes are 
present together, causing the intermediate condition of hybrids. 
In the second case, on the other hand (where the entire gene 
varies In amount), no perceptible difference in effect is ordinarily 
produced, as seen by the similarity in the expression of most 
sex-linked factors in male and female. This unlikeness between 
the results of quantitative changes in a part of a gene and in the 
whole gene would presumably be because the parts of a single 
gene have such a direct and intimate connection with each 
other in the reactions of morphogenesis that the quantitative 
relations of their parts make a great difference in development, 
whereas the evidence shows that separate genes usually interact 
in a different, less direct manner, such that their relative amounts 
(within + 50 per cent at least) make little difference. Here, 
then, we would have a basis for defining the limits of a gene, in 
addition to the test of crossing over. 
