RELATIVE GROWTH AND GENETICS 231 



The second point, the slight lightening of eye-colour after 

 maturity, is due to the expansion in size of the individual 

 eye-facets, causing the same amount of pigment to be spread 

 over a greater area (Ford and Huxley, 1927). 



This complex relationship is precisely similar to what we 

 have found to obtain for the relative growth of an organ 

 such as a limb (see Chapter VI), where two factors are also 

 at work, the inherent growth-potential of the organ, and some 

 mechanism regulating this in relation to the growth of the 

 body as a whole. Since the relative factor occurs in both 

 cases, it is not to be expected that the curves for characters 

 controlled by rate-genes will shed light directly on the nature 

 of the biochemical processes involved, even if plotted against 

 time. It would be theoretically better to plot them against 

 some quantitative character of the animal, e.g. the amount 

 of some important substance in the body, or even the total 

 body-bulk. In this way we should obtain a growth-partition 

 coefficient for the chemical substance responsible for the 

 character, just as we obtain a similar coefficient for the growth 

 of heterogonic organs when their size is plotted, not against 

 time, but against body-size ; and it is these coefficients which 

 are of the greater biological importance, and are quantitatively 

 comparable one with another. That this method may be 

 valuable is seen in the already cited work of Teissier (1931). 



The second general point is this. Just as the proportions 

 of a fiddler-crab are continuously changing, so may the eye- 

 colour of a Gammarus change throughout life, or at least 

 until long after sexual maturity. But a holometabolous insect 

 has its adult proportions fixed once for all at the attainment 

 of the imaginal instar ; and similarly even characters that 

 are determined by rate-factors will in holometabolous insects 

 become fixed at the same period — the final moult will take 

 a cross-section across the developmental curves whose results 

 will only be made visible at this instant. Thus a static is sub- 

 stituted for a dynamic picture. 1 



We may thus suppose that such a gradation as that presented 

 by the eye-colour of various multiple allelomorphic series 

 (e.g. of the white-eye locus in Drosophila) represents merely a 



1 This is not rigidly true : there are, for instance, some eye-colours 

 in Drosophila which change with the age of the imago, but such changes 

 play a minor part as compared with those in, e.g., Gammarus, and it 

 is quite possible that they are not of the same nature ; in any case they 

 are not known to be related to size. 



