236 



PROBLEMS OF RELATIVE GROWTH 



Since both the rate of production of the substance and the 

 precise time of differentiation can be varied both by environ- 

 mental and genetic agencies, there is room for considerable 

 elasticity in the end-result provided that this is between the 

 normal upper and lower thresholds for the development 

 of the character in question. E.g., in normal male Lyman- 



tria an adequate 

 excess of male- 

 determining over 

 female - determin- 

 ing substance is 

 present well before 

 the period of 

 differentiation; 

 but in female 

 intersexes this ex- 

 cess of male-deter- 

 mining substance 

 i s transformed 

 into an excess of 

 female - determin- 

 i n g substance 

 during the differ- 

 entiation phase. 

 Thus whereas 

 normal males are 

 not affected as 

 regards their sex- 

 characters by en- 

 vironmental agen- 

 cies such as 

 t emperature, 

 similar agencies 

 applied during the 

 development o f 

 an intersex can 

 exert a marked 

 influence on the degree of intersexuality. 



So with vestigial organs (Fig. 102). If the production of a 

 limb-determining substance, for instance, be normally above 

 the necessary threshold well before the onset of the phase of 

 differentiation during metamorphosis, the limbs will be stable 

 organs, little affected in their proportions or characters by 



Fig 



102. — Hypothetical curves to illustrate the 

 relation of rate-genes to vestigial organs. 



1 — 1 , lower threshold of amount of determining substance which 

 must be produced by the rate-gene before differentiation of the 

 organ can begin. 2 — 2, upper threshold ; if this is reached during 

 the period of differentiation, complete development of the organ 

 occurs. X— X, effective time of differentiation. This may be 

 varied so as to occur earlier (X~) or later (X + ). A — A, action of 

 rate-gene promoting normal complete development of organ ; the 

 full effective amount of determining substance is always produced 

 before 'the period of differentiation. B — B, action of rate-gene 

 whose intensity has been reduced to produce the total evolutionary 

 disappearance of the organ. The lower threshold of determining 

 substance is never reached before the period of differentiation. 

 C — C, action of rate-gene normally promoting a vestigial develop- 

 ment of the organ. Slight variations in its intensity or in the time 

 of differentiation will produce large differences in the degree of 

 development of the organ. 



