THE NATURE OF THE GENE 287 



of the gene; but in no other cases can it be proved that muta- 

 tional changes are of a quantitative nature. This conclusion, 

 of the agnostic type, is unassailable, as agnosticism usually is, 

 but it is not constructive. 



2. From an evaluation of the facts concerning pleiotropic 

 action of genes (Dobzhansky, 1930; Muller, 1932; Stern, 1930). 

 These facts were discussed (see page 89), and it was shown that 

 there is no reason to assume that very different developmental 

 processes must be affected in a perfectly parallel plus-minus 

 seriation, as threshold conditions and numerous interactions 

 come into play. A perfectly parallel change in regard to different 

 processes, affected by the same gene and its allelomorphs, is 

 actually expected to be the rarer occurrence. Wright (1916), who 

 found the first case of nonparallelism, realized that the explana- 

 tion has nothing to do with the gene but with the genetic and 

 other environment in which different developmental reactions 

 take place (see page 88). 



3. The most serious general criticism of the theory discussed 

 here is that it is very difficult to imagine a physicochemical 

 mechanism that controls the constancy of the number of gene 

 molecules in the different loci of a chromosome. Goldschmidt 

 (1917a) had pointed to some possibilities, but their realization 

 would require very difficult assumptions indeed. Later, (1932a) 

 Goldschmidt returned to this problem and pointed out that new 

 developments of chemistry could supply a better model. To this 

 point we shall return later (see page 293). 



We may draw the following conclusions: 



1. Actually genes may act in proportion to their quantity, and 

 this relation is, ceteris paribus, of the same type as that found 

 in the kinetics of chemical reactions. 



2. In some cases, what has been described as different allelo- 

 morphic conditions of genes has turned out to be a series of 

 actual quantities of chromosome material (Bar, deficiencies). 



3. In some cases, the facts permit the inference that gene muta- 

 tions are actually changes in the dosage of the gene. 



4. The conclusion that mutations are generally of the type of a 

 quantitative change in gene substance would have to be supple- 

 mented by the still missing demonstration of the mechanism to 

 control constant quantities. Modern developments of the gene 



