g.| The sexual Organization of the Rhizocephala. 



unknown, save perhaps in Pollicipes and such a form as Balanus perforatus; but here, as Gruvei. 

 has pointed out (15 p. 431), cross -fertilization may be effected by means of giant wandering 

 spermatozoa. 



The capacity for self-fertilization (see 2 and 3) appears to arise in an animal or plant 

 as though by a capricious chance ; thus scattered among the genera of certain groups of plants, 

 especially the Orchids, we may find single species as for instance Ophrys apifera, which have 

 given up the process of cross-fertilization for which they were evidently originally intended, 

 and have taken to continuous self-fertilization without apparent damage; while in Darwin's 

 classical experiments on the effects of cross- and self-fertilization in plants, the self- fertile 

 varieties of plants which are generally adapted for cross-fertilization, e. g. races of Mimulus 

 luteus, Nicotiana and Reseda, appeared to arise by the merest caprice. 



We can only in our ignorance suppose that certain ancestral varieties of the Rhizo- 

 cephala were by chance self-fertile, and that owing to their being solitary parasites, this method 

 of propagation was perpetuated by natural selection as being the only one suited to the con- 

 ditions of the animals lives. But this does not in the least explain how the evil effects of 

 self-fertilization are obviated, or what these evil effects really are. 



But not only have the Rhizocephala adopted perpetual self-fertilization, but they have 

 done so when a means of cross -fertilization was apparently at hand, namely by the comple- 

 mental males. "We must now attempt to give some explanatory account of these organisms. 



In order to do so it is necessary to consider shortly the sexual variations of the Cirri- 

 pedia in general, since the Rhizocephala are evidently derived from some ancestral form 

 common to the rest of the Cirripedia. 



The majority of the Cirripedia. i. e. the majority of the Pedunculata, and all the Oper- 

 culata are simple cross-fertilizing hermaphrodites: in the Abdominalia and Ascothoracica, the 

 sexes are separate, but it is in certain Pedunculate forms that the most interesting conditions 

 are found. As originally pointed out by Darwin (1 , and confirmed by Hoek (8) and Gruvel 

 (11 and 15), there are two main variations of the ordinary plan: firstly we may have herma- 

 phrodites to which complemental males are attached as in Scalpellum vulgare, the majority of 

 the species of Scalpellum, and Ibla cumingii; or else females to which small males, resembling 

 coiuplemental males more or less, are attached as in Scalpellum velutinum and ornatum and 

 Ibla quadrivalvis. 



The ordinary view of these variations, the view adopted by the authors cited, is that 

 they have been derived from the ordinary hermaphrodite condition found in most Pedunculata, 

 and that we are witnessing in them stages in the secondary separation of the sexes 

 from the hermaphrodite state. 



The occurrence of complemental males in a degenerate condition in the Rhizocephala 

 is a serious, if not fatal, objection to this view, because we must suppose that the Rhizo- 

 cephala are an ancient group of animals and that they diverged from the other Cirripedes at 

 a remote period: if then the complemental males are a progressive stage in the secondary 



