82 2 ORIGIN OF SPECIES. 



them ; and they are usually very variable. Wichura showed, from his own observ- 

 ations and those of Gartner, that hybrid pollen produces a greater variety of forms 

 in its progeny than does the pollen of true species. 



The results of hybridisation are important with respect to the theory of sexuality, 

 because there is no boundary-line or essential distinction between the self-fertilisation 

 of pure species or varieties and their fertilisation by other species or varieties ; and be- 

 cause in the latter case — in other words in hybridisation — certain peculiarities of sexual 

 differentiation and union are rendered more evident. The two extremes of the con- 

 ditions under which a fertile union of sexual cells is possible lie at a great distance from 

 one another, but are connected by very numerous transitions. One extreme is presented 

 in the genus Rhynchonema and in some Saprolegnieae, where a fertile sexual union of 

 sister-cells takes place regularly ; the other extreme is furnished in genus-hybrids, where 

 the uniting cells belong to very different forms of plants whose descent from a common 

 ancestor dates back to a remote antiquity. But the great majority of phenomena in the 

 vegetable kingdom show that sexual union is usually most productive when the cells 

 stand neither in too close nor in too remote an affinity to one another ; self-fertilisation 

 is in the vast majority of cases as carefully avoided as the hybridisation of different 

 species or genera. The phenomena may be comprised in the statement that the original 

 form of sexual differentiation was probably the simultaneous formation of male and 

 female organs in close juxtaposition on the same plant, but that sexual union is more 

 potent and more favourable for the maintenance of the race when the closely contiguous 

 cells do not unite, but those of different descent, a certain mean amount of difference of 

 descent being established as the most favourable. This mean of the difference of descent 

 associated with a maximum of sexual potency is obtained when the sexual cells belong 

 to different individuals of the same species \ The phenomena of structure described 

 in the preceding paragraphs which are manifested in polygamy, diclinism, dichogamy, 

 dimorphism, the impotence of pollen on the stigma of the same flower (as in Corydalis 

 and Oncidium), the mechanical contrivances for rendering self-fertilisation impossible 

 (as in Aristolochia Clematitis, many Orchidess, &c.), are different means for promoting 

 the cross-fertilisation of individuals belonging to the same species or for rendering it 

 alone possible. 



CHAPTER VII. 

 THE ORIGIN OF SPECIES. 



Sect. 33. — Origin of Varieties. The characters of plants are transmitted to 

 their descendants, or, in other words, are hereditary. But, in addition to the inherited 

 properties, new characters may arise in a smaller or larger number of the descend- 

 ants of a plant which were not possessed by the parent-plants. Thus, for example, 

 Descemet obtained in 1803^, among the seedlings from Rohinia Pseud-acacia, an 



^ [See Darwin, Variation of Animals and Plants under Domestication, vol. II, chap, xvii, where 

 several illustrations of the law are given. — En.] 



' See Chevreul, Ann. des sci. nat. 1846, vol. VI, p. 157. [Journ. Roy. Hort. Soc. vol. VI, 

 1851. p. 61.] 



