I'M) PHYSIOLOGICAL GENETICS 



of the gene consist in changes in its quantity, say from 5 to 6 or 

 to 10 molecules or from ") to 4 or to 2, etc., molecules. This. 

 again, presupposes that the quantity of a gene (the number of 

 molecules) is as constant as its quality and that a mutational 

 change in quantity leads to a new equilibrium. 



This theory has been criticized with a number of objections, 

 many of which are not relevant because based upon misunder- 

 standings. To mention only one: it has been argued (Muller, 

 1932a; Timofeeff, 1933a) that the existence of reverse mutations 

 excludes the quantity idea. It is difficult to imagine what is 

 actually meant by such an argument. If it is possible that the 

 typical quantity of gene molecules is controlled by the underlying 

 chromosome mechanism, and if gene substance is produced as a 

 result of gene action (autocatalysis), an idea without which no 

 theory of the gene can work, the number of these molecules may 

 increase or decrease, whenever the equilibrium is disturbed (this 

 is the meaning of mutation). 



More serious are the following objections: 



1. From the standpoint of chemistry. It was pointed out by 

 Wettstein (1924a) that typical changes in the velocities of 

 reactions could be produced, e.g., by a change in pH. Schmalfuss 

 (1928J7.) performed many model experiments with melanin 

 formation and showed that models can be made up in which 

 differences in the quality of the chromogen or the oxydase might 

 lead to different quantitative results in regard to melanin forma- 

 tion. The correctness of such statements is obvious and coin- 

 cides, after all, w r ith everyday experience. If, for example, 

 growth is controlled by a vitamin and by a hormone, their com- 

 bined effect is quantitative. Multiple factors, which probably 

 are different in quality, also combine to make a quantitatively 

 different action. But such facts have nothing to do with those 

 cases from which the theory has been derived, in which some 

 members of the 1 series are actual quantities, a fact from which 

 an inference is extended to the other members. The only 

 conclusion that may be drawn from criticism of the type here 

 discussed is that many cases are known in which the gene acts 

 proportionally to its actual quantity. Also, there are many 

 other cases in which it is possible to draw an inference from the 

 knowledge of the action of actual gene dosage upon intermediate 

 effects of other gene conditions as due to intermediate quantities 



