246 HOMOLOGICAL RELATIONS OF THE 



generative gland first appears, there are distinct foci of de- 

 velopment for both sexual elements — though the normal 

 development of one suppresses that of the other ; — or we 

 may adopt the more common theory, that there is but a 

 single focus of this kind, whose development is, in some un- 

 known way, directed exclusively to the evolution of one or 

 other sex. The latter view, certainly, is that most naturally 

 suggested by the facts of the case in the higher species, and 

 also by the development of the gonophores of the Hydrozoa. 

 In the lower animals, generally, the primordial condition of 

 these organs has not yet been much studied in this point 

 of view. 



"Wlien the sexes are separate, two individuals — male and 

 female — are required for an adequate representation of the 

 species ; but, in cases where they are united, each indivi- 

 dual of the species becomes a complete type of the whole, 

 so far as structure is concerned. Functionally, however, 

 it is only so, when the bi-sexual organism has the power of 

 self-impregnation ; and this is comparatively seldom the 

 case, most hermaphrodites impregnating each other, either 

 by a reciprocal act (as in Helix), or by their aggregation 

 into chains, the individuals composing which receive fecun- 

 dation from those in advance, and communicate it to those 

 behind.* 



On the other hand, in the cases in which the oneness of 

 the organism is broken in upon by an interpolated process 

 of gemmation, the sexual characters attach themselves ex- 



* In some bi-seraal Cin-ipedes, it lias been shown by Mr. Darwin that a 

 sort of supplementary impregnation is effected by separate males of mi- 

 croscopic size and parasitic habits, which he terms complementary males. 



In aquatic animals, intromission of the spermatic particles is by no 

 means so regular a phenomenon as in terrestrial species, impregnation 

 being frequeutly effected by the diffusion of the seminal fluid through 

 the water in the neighbourhood of the previously deposited spawn of the 

 female. The process may be imitated artificially, and this is now done to 

 some extent in the breeding of fish. 



