7 8 THE EVOLUTION OF SEX. 



Por it seems plausible that, with more available material for 

 internal differentiation, such should actually occur. But it is 

 possible to venture still further. 



A sluggish habit is usually associated with a large surplus of 

 nutritive material, and at the same time very frequently with an 

 accumulation of waste products. Parasitism means not only 

 abundant, but rich and stimulating nutrition. Conditions 

 which combine these two factors w^ill tend to secure the persist- 

 ence 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 characterised by hermaphroditism. 



§ lo. Origin of Hermaphrodiiism. — There can be very little 

 doubt that hermaphroditism was the primitive state among 

 multicellular 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 opinion in the following cautious 

 and terse words : — " The hermaphrodite stage is the lower, 

 and the condition of distinct sexes has been derived from it." 

 Unisexual "differentiation, by the reduction of one kind of 

 sexual apparatus, takes place at very different stages in the 

 development 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 restric- 

 tion of areas, in which the formation of two kinds of cells still 

 went on at different times in one organism. In different in- 

 dividuals the opposite tendencies we have already spoken of 

 more and more predominated, till unisexuality evolved out of 

 hermaphroditism. 



We may in l:)rief suggest as the three probal^le grades in the history : — 

 {a) The liberation of unindividuated sex-elements ; (/») 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 ; (r) the 

 unisexual outcome, where the production of one set of elements has pre- 

 ponderated over that of the other. 



As at present existing, hermaphroditism may be interpreted 



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



Individual cases must be judged by themselves, and the history 



