2 3 8 



PROBLEMS OF RELATIVE GROWTH 



relative length of the fore-limb increases between fetus and 

 adult. This happens to be ' recapitulatory ' in excessively 

 long-armed forms like the gibbon, but is the reverse of reca- 

 pitulatory in man, who is undoubtedly descended from brachi- 

 ating ancestors with relatively longer arms. The great 

 similarity of the fetuses is due to the fact that in all cases 

 the arms begin relatively small, and that final differences of 

 proportion are due to the different degrees of positive heter- 

 ogony which they exhibit. The ' recapitulation ' by the 

 new-born lamb in improved breeds of sheep of the approximate 

 proportions of adult wild sheep (Chapters III, VII) is another 



%13 

 %12 



hi 



%10 



I 9 

 \ * 



I ? 

 |* 



4r « 



•5 •» 



t 2 



Extrusion 



8 

 Time in Weeks 



10 tl 12 



Fig. 103. — Effect of temperature on eye-darkening in a pure genetic 

 strain (rrSS) of Gammarns chevreuxi. 



At io° C. no melanin is deposited. At 13°, deposition begins only after 5-6 months. The figure 

 shows the facts for temperatures from 15 to 28°. 



example of a change of this sort which happens to be reca- 

 pitulatory. 



All coloured Gammarus eyes so far investigated begin as 

 pure red (Ford and Huxley, op. cit.). But we have in this 

 no grounds whatever for supposing that Gammarus are de- 

 scended from a red-eyed ancestor. It would appear to be a 

 direct consequence of the empirical fact that melanin deposi- 

 tion can only begin after the red (lipochrome) pigment has 

 been formed. 



Again, the fact that both chelae of male Uca begin of female 

 type, so that males and females are more alike when young, 

 does not in the least prove that the males of any ancestral 

 Uca ever possessed two chelae of existing or any other female 

 type. In fact, all the probabilities are against this. It is due, 



