THE M I TA TED GENE 53 



of orderly but varying velocities were involved. Sex genes, then, 

 were conceived as controlling the rate of production of sex- 

 determining stuffs. From these two sets of facts and their 

 genetic and embryological analysis, a generalization was derived 

 applying the principle of rate to all genie actions. (For details 

 see Goldschmidt, 19206,c, 1927c.) 



Many cases have since been studied which show a connection 

 between mutant genes and rates of processes. In this chapter 

 only certain cases will be mentioned in which the rate concept 

 has not been inferred from the facts but where actual rates have 

 been measured. Ford and Huxley (1927) were able to measure 

 the rate of pigmentation of the eye of Gammarus chevreuxi. In 

 the normal dominant Black-eyed form, the eye pigment first 

 appears scarlet. It later darkens through brown and chocolate 

 into black. In the Red-eyed mutation, the darkening of the pig- 

 ment takes place much more slowly and also starts later. The 

 end of the process, therefore, reaches a stable phase before black- 

 ening is completed. These two different rates have been plotted 

 as curves and have been found to depend to a certain extent 

 upon external conditions. Genetically the genes involved con- 

 trol either the rate or the onset of the pigmentation process. 



Very nearly related to the facts concerning pigmentation in 

 Lymantria caterpillars are those found by Goodrich and Hansen 

 (1931) for pigmentation in fishes. They studied the increase 

 in the number of pigment cells in brown and transparent goldfish 

 in the course of development (genes T and 7\). Figure 17 shows 

 the rate curves for the two homozygous and the heterozygous 

 forms which resemble closely Goldschmidt's curves for pigmenta- 

 tion of Lymantria. In this category would probably also belong 

 the results of Hashimoto (1936) with the silkworm if an embryo- 

 logical study had been made. There is a mutant gene in this 

 form causing an extra pair of abdominal feet. In heterozygous 

 condition, only the feet are produced; in homozygous condition, 

 the next segment is also influenced and has an extra semilunar 

 spot. In this connection, such gene actions might also be 

 mentioned as the genes controlling later or earlier growth of 

 feathers in the chicken according to Serebrowsky and Warren. 



A whole group of cases is known in which the mutant gene 

 affects definite growth rates. Of course, in this case, one might 

 claim that there is no difference between the action of a gene 



