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 dicecious (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 moneecious (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 dicecious 
species that are hermaphroditic; and, conversely, individuals of 
separate sexes sometimes appear in moncecious species. The 
prevailing 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 moncecious 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 fertiliza- 
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 possibility that there might be male and fe- 
male lines 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. 
