920 ORIGIN OF SPECIES. 



The results of hybridisation are important \yith 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 

 because 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, 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 arrangements described in the pre- 

 ceding paragraphs which are manifested in polygamy, diclinism, dichogamy, dimorphism, 

 the impotence of pollen on the stigma of the same flower (as in Corjdalis and 

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

 in Aristolochia Clematitis, many Orchidese, &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. 35. — Origin ofVarieties. 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 descendants 

 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 

 individual without spines; Duchesne, in 1761^, among seedHngs of the Strawberry, 

 one with simple instead of trifoliolate leaves ; and Godron*, among seedHngs of 

 Datura Taiula, one with smooth instead of spiny capsules. 



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

 several illustrations of the law are given.]] 



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

 1851, p. 61.] 



2 For further details, See Usteri, Annalen der Botanik, vol. V. p. 40. 



* See Naudin, Compt. rend. 1867, vol. LXIV, p. 929. 



