364 Experimental Zoology 



Individuals of two kinds exist in many species of animals, — 

 males and females. Such individuals are said to be uni- 

 sexual, and the species dioecious (or of two households). 

 In other species there is only a single kind of individual 

 that produces both eggs and spermatozoa. Such individ- 

 uals are bisexual or hermaphroditic; and the species is said 

 to be monoecious (or of one household). Closely related species 

 may belong to the one or to the other of these two groups, so 

 that the distinction does not appear to be of fundamental im- 

 portance. Moreover, individuals sometimes appear in dioecious 

 species that are hermaphroditic; and, conversely, individuals of 

 separate sexes sometimes appear in monoecious species. The 

 prevaihng view is probably correct, that male individuals carry 

 in a latent or potential condition the female characters, and the 

 female those of the male. If the latent character develops, an 

 hermaphrodite appears; and if, in a monoecious species, one 

 set of characters fails to develop, a male or a female appears. 

 From this point of view we can readily understand how easily 

 the transition from one to the other kind of sexual individual 

 may take place. 



A third kind of individual is also recognized, namely, the par- 

 thenogenetic. Such an individual is looked upon as a female, 

 in which the eggs have the power to develop without fertilization. 

 This may sometimes happen in ordinary females, so that par- 

 thenogenetic reproduction is not sharply separated from sexual 

 reproduction. Even in the same individual, as we have seen in 

 the case of the bee, the eggs may develop with or without fertiHza- 

 tion. Since parthenogenetic individuals may produce males as 

 well as sexual females, we must conclude that the male charac- 

 ters are carried in a latent condition by these parthenogenetic 

 females, often through a long series of purely parthenogenetic 

 generations. The possibihty that there might be male and fe- 

 male hncs of such parthenogenetic females is excluded by find- 

 ing that the same individual may produce both males and 

 females, as seen in aphids and daphnia. 



