74 THE EVOLUTION OE SEX. 



tention. For it seems plausible that, with more available material 

 for internal differentiation, such should actually occur. But it is pos- 

 sible to venture still further. 



A sluggish habit is usually associated with a large surplus of nutri- 

 tive material, and at the same time very frequently with an accumula- 

 tion of waste products. Parasitism means not only abundant but 

 rich and stimulating nutrition. Conditions which combine these two 

 factors will tend to secure the persistence of primitive hermaphroditism, 

 or even to develop it from a previously attained unisexual state. It 

 must be noted, however, that exceptions occur, which it is at present 

 difficult to explain. The ctenophores are all hermaphrodite, yet very 

 active. So, too, are not a few tunicates; while the brachiopods are 

 extremely passive, but not specially characterized by hermaphroditism. 



X. Origin of Hermaphroditism. — There can be very little 

 doubt that hermaphroditism was the primitive state among multicel- 

 lular animals, at least after the differentiation of sex-elements had been 

 accomplished. In alternating rhythms, eggs and sperms were produced. 

 The organism was alternately male and female. Of this primitive 

 hermaphroditism, there is probably more or less of a recapitulation 

 in the life-history of all animals. Gegenbaur states the common opin- 

 ion in the following cautious and terse words: "The hermaphrodite 

 stage is the lower, and the condition of distinct sexes has been denved 

 from it." Unisexual "differentiation, by the reduction of one kind 

 of sexual apparatus, takes place at very different stages in the develop- 

 ment of the organism, and often when the sexual organs have attained 

 a very high degree of differentiation." The first structural stage in 

 the separation would probably be the restriction of areas, in which the 

 formation of two kinds of cells still went on at different times in one 

 organism. In different individuals the opposite tendencies we have 

 already spoken of more and more predominated, till unisexuality 

 evolved out of hermaphroditism. 



We may, in brief, suggest as the three probable grades in the 

 history: — (a) The liberation of unindividuated sex-elements; (<5) the 

 formation of two diverse kinds of sex-elements, incipiently male or 

 female, at the same time or at different periods, according to nutritive 

 and other conditions; (V) the unisexual outcome, where the production 

 of one set of elements has preponderated over that of the other. 



As at present existing, hermaphroditism may he interpreted as a 

 persistence of the primitive state, or as a reversion to it. Individual 

 cases must be judged by themselves, and the history of each must be 

 taken into account. But where the hermaphroditism is manifestly 

 exceptional, there can be seldom any question in regarding it as a 

 reversion. The reversion would generally occur on the female side, 



