RELATIVE GROWTH AND GENETICS 229 



highly probable that the size of tendons in the mammalian 

 body is entirely or almost entirely due to functional response. 

 Thus, when we compare organs of different relative size in 

 two individuals, races, or species of different absolute size, 

 our analysis should be devoted to establishing (1) whether 

 the difference is due to the organ's capacity for functional size- 

 regulation ; (2) whether it is a mere consequential effect of 

 increase of absolute size, as is the case with the relatively 

 large antlers of Red Deer imported into New Zealand from 

 Scotland, or of the relatively large mandibles of absolutely 

 large male Lucanidae ; or (3) whether it is due also to genetic 

 alteration in the growth-partition coefficient ; or (4), of course, 

 to a combination of these in differing degrees. 



§ 5. Relative Growth and Genetics 



We next come to the subject of genetics. It is clear that 

 the chief genetic factors controlling any organs which show a 

 constant differential growth-ratio or a constant growth- 

 partition coefficient must be rate-genes, or genes which deter- 

 mine the rate of a developmental process. Such factors have 

 been deduced or demonstrated for numerous characters, 

 notably in the sex-determining genes of moths (Goldschmidt, 

 1923, 1927) and the eye-colour genes of the crustacean 

 Gammarus (Ford and Huxley, 1929) (for other cases see the 

 references in the works cited, and also Ford, 1929, Belehradek 

 and Huxley, 1930) ; see Figs. 98, 99. 



There are two general points which are worth stressing. 

 The only case where the curve for the development of such 

 characters has been quantitatively obtained by direct measure- 

 ment is that of the eye-colour of Gammarus. Here it appears 

 clear that the actual form of the curve is determined by two 

 factors — an inherent greater or less velocity of pigment- 

 deposition, and some regulating mechanism which relates 

 this to the growth of the body as a whole. This is clearly 

 brought out by two facts — first, the relations in slow-growing 

 mutants ; secondly, the slight lightening of colour which 

 occurs in the eyes of certain strains of Gammarus after sexual 

 maturity. ' Slow growth ' is a recessive mutation, which at 

 first shows abnormally dark eyes. This appears to be due to 

 the fact that the eggs are of normal size, that a pigment 

 precursor is formed in them in some simple relation to egg- 

 size, and then, when the time comes for the deposition of 

 pigment, the eye-area, over which it is to be spread, is abnor- 



