DIMORPHISM AND POLYMORPHISM 367 



The ids of the germ-plasm must contain more determinants 

 in sexually dimorphic than in monomorphic species, the number 

 increasing in proportion to the increase of difference between 

 the sexes. They must also increase in size ; and the question 

 therefore arises as to whether dimorphism may not perhaps 

 increase to such an extent and finally involve all parts of the 

 body, that double ids arise ; that is to say, each id of the germ- 

 plasm comes to consist of a male and a female half, in which 

 all the determinants are different. This seems to be practically 

 the case in some animals : in the Rotifera, for instance, the 

 males commonly differ so much from the females that they, 

 like the sexual form of Phylloxera, arise from special eggs, 

 smaller than those which develop into females. But it would 

 nevertheless be incorrect to suppose that each of these two 

 kinds of eggs contained either male or female ids only. The 

 number of common determinants present must certainly be 

 small, but even here the germ-plasm of each ^gg must never- 

 theless contain all the male and female determinants. This is 

 proved by the interpolation of generations consisting of females 

 only in the cycle of generations which is passed through each 

 year, the parthenogenetic females eventually producing males. 



We must now return to the question of sexual reve?'sio7i — if I 

 may so call it — which has already been referred to ; that is to 

 say, the appearance of characters of the opposite sex after cas- 

 tration or degeneration of the sexual glands. Hitherto this has 

 always been considered a universal phenomenon, but I do not 

 think that such a conclusion is justified. As already mentioned, 

 observations w'ith regard to such cases of ' sexual reversion ' 

 have practically been confined to birds and mammals, and even 

 in these do not always refer to all the parts which are sexually 

 dimorphic in the species in question. Cases have certainly been 

 observed in which, for instance, an old ' hen which had ceased 

 laying, assumed the plumage, voice, spurs, and warlike dis- 

 position of the cock.' * This proves that in these birds all the 

 secondary sexual characters of the male are present in a latent con- 

 dition in the soma of the female. We might, however, suppose 

 that this form of reversion only takes place when the characters 

 concerned are completely homologous in the two sexes ; that is 



* Darwin, ' Animals and Plants under Domestication," Vol. II., p. 26. 



