920 
ORIGIN OF 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 
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 Corydalis 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 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 descendants 
of a plant which were not possessed by the parent-plants. Thus, for example, 
Descemet obtained in 1803^, among the seedhngs from Rohinia Pseud-acacia, an 
individual without spines; Duchesne, in 1761^, among seedlings of the Strawberry, 
one with simple instead of trifoliolate leaves ; and Godron*, among seedlings of 
Datura Tatula, 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 sei. nat. 1846, vol. VL p. 157. [Journ. Roy. Hort. Soc. vol. VI, 
1851, p. 61.] 
^ For further details, See Usteri, Annalen der Botanik, vol. V. p. 40. 
* See Naudin, Compt. rend. 1867, vol. LXIV. p. 929, 
