Nos. 622-623] THE SEXES IX THE AVE ID 



517 



the other, not an intermediate, it is hardly possible to 

 escape the conclusion that the thing which brings about 

 or prevents the morphological alteration is a gradual 

 process. What this gradual change may be in the present 

 case can not be known from the evidence, for obviously 

 changes in the type of metabolism may be of various 

 kinds. 



Riddle (1917) conceives of such a change of metabolism 

 as a change from individuals having a high rate of metab- 

 olism and low energy content to individuals having a 

 low rate of metabolism and high energy content. In the 

 eggs of pigeons forced to lay eggs continuously, he finds 

 just such a change. The early eggs are of the former 

 type, the late eggs of the latter type. From the early 

 eggs are developed males, from the late ones females; 

 and on those facts, supported by other work, Eiddle 

 bases an elaboration of the Geddes and Thompson theory 

 of sex. 



An attempt has been made to fit the facts obtained from 

 aphids to Riddle's conception of sex. The gradual 

 transition that occurs both in the parthenogenetic and in 

 the sexual phase of the cycle of Macrosiphum indicates 

 that one type of metabolism is prevalent early in the 

 cycle, and the contrasted type late in the cycle. The fact 

 that in this transition males precede sexual females shows 

 that, if Riddle's hypothesis holds for the aphids, the pro- 

 gressive change is from a high rate of metabolism and 

 low energy content to a low rate of metabolism and high 

 energy content. Now it has been shown that wingless 

 viviparous females precede winged ones, the change from 

 the one form to the other taking place in part simulta- 

 neously with the transition from males to sexual females. 

 Hence in accordance with Riddle's scheme wingless 

 females should represent a high rate of metabolism and 

 low energy content, while the winged ones should possess 

 a low rate of metabolism and high energy content. With 

 regard to the rate of metabolism alone, this assumption is 

 supported bv the fact that winged females require longer 



