RELATIVE GROWTH AND RECAPITULATION 237 



environmental variations. Equally, if the rate-genes con- 

 cerned are so much reduced in intensity that the limb-determin- 

 ing substances are well below the threshold during the whole 

 differentiation period, the absence of limbs will be a stable 

 character. But if the intensity of the rate-genes has only been 

 reduced to a pitch at which a certain small amount of limb 

 development can normally proceed, environmental agencies 

 will quite certainly be capable of affecting the precise degree 

 of this development within fairly wide limits. A good example 

 of variability of this sort in vestigial organs is provided by the 

 vestigial limbs of the stump-legged mayfly — Campsurus segnis, 

 described and figured by A. H. Morgan (1929). 



A somewhat similar fact, due to somewhat similar causes, 

 is found in Gammarus (op. cit.). Normal black eyes, which 

 are due to genes whose influence is to take melanin formation 

 very rapidly up to saturation-point, are scarcely affected by 

 temperature. Other genes slow down the process so much 

 that the threshold for visible melanin-production is normally 

 never reached. The permanently pure red eyes thus produced 

 also are little affected by temperature. Genes of intermediate 

 strength, however, which at room temperature produce a 

 moderate darkening to red-brown, are markedly influenced 

 in their effects by temperature-changes : at high temperatures 

 the eyes of animals containing such genes are dark chocolate, 

 at low temperatures almost pure scarlet (Fig. 103). 



Finally, since rate-genes can obviously mutate both in the 

 plus and the minus direction, so as to accelerate or slow down 

 the processes which they control, it is clear that changes in 

 rate-genes could as easily lead to the opposite of recapitulation 

 as to recapitulation. Many examples of neoteny would fall 

 under this head. See e.g. Mjoberg's discussion (1925) on the 

 larviform females of certain Lycid beetles. De Beer (1930) 

 has dealt fully with this point, and has proposed a useful 

 terminology. 



Finally, it is clear that numerous examples of von Baer's 

 Law, that related forms tend to resemble each other more in 

 the early stages of their development than when adult, need 

 have nothing whatever to do with Haeckel's reformulation 

 of von Baer's Law, or with recapitulation, but are simply 

 consequences of the laws of relative growth and of physio- 

 logical genetics. For instance, the proportions of the fore- 

 limbs of man and all anthropoids are more similar in the fetus 

 than in the adult (Schultz, 1926). But in every case the 



