THE HEREDITARY FACTORS AND DIFFERENTIATION 409 



As to the intermediate steps in the chain of processes by means 

 of which the hereditary factors influence differentiation, Httle is 

 known. It is, however, becoming clear that many genetic differ- 

 ences, including certain apparently qualitative effects, depend upon 

 quantitative differences in the rate of action of the factors. The 

 hereditary control of the rates of certain developmental processes 

 has been studied in the insect Lymantria, and in the crustacean 

 Gammarus. 



In Lymantria it has been shown that sexual differentiation is 

 conditioned by a competition between two sets of processes : those 

 controlling the production of structures that characterise the female, 

 and those which characterise the male. These in their turn are con- 

 trolled by hereditary factors, the female-determiners which seem 

 to be lodged mainly in the F-chromosome, and the male-deter- 

 miners lodged in the X-chromosome. In normal development, one 

 or another of these sets of processes wins before the time at which 

 differentiation takes place, but, by making appropriate crosses 

 between individuals of different races, and in pure strains under 

 extreme experimental conditions, it is possible to alter the circum- 

 stances in such a way that an animal will develop along the female 

 line up to a certain point, but is then switched over to the male type, 

 or vice versa. The sooner this switching over takes place, the more 

 complete is the sex-reversal.^ 



In Gammarus, it has been demonstrated that the difference be- 

 tween adult black, chocolate, red-brown, and red eye-colour, is an 

 effect of quantitative differences in the rate of deposition of me- 

 lanin pigment in the facets of the eye, and that these differences are 

 controlled by hereditary factors.^ The interaction between genetic 

 factors and environment to produce a given character is also here 

 very well illustrated. At normal temperatures, the "rapid-darken- 

 ing red" factor or gene produces adult chocolate eyes. But at 

 temperatures below a certain threshold, no melanin at all is pro- 

 duced, and the eyes remain pure red. At intermediate temperatures, 

 intermediate shades of adult eye-colour are produced. One and 

 the same gene leads to different rates of melanin-formation in 

 ^ Goldschmidt, 1927. ^ Ford and Huxley, 1927. 



