Cross-fertilisation 417 



fluenced by external conditions. Even individuals growing close to 

 one another are only apparently exposed to identical conditions. 

 Their sexual cells may therefore be differently influenced and thus 

 give favourable results on crossing, as "the benefits which so 

 generally follow from a cross between two plants apparently depend 

 on the two differing somewhat in constitution or character." As a 

 matter of fact we are familiar with a large number of cases in which 

 the condition of the reproductive organs is influenced by external con- 

 ditions. Darwin has himself demonstrated this for self-sterile plants, 

 that is plants in which self-fertilisation produces no result. This 

 self-sterility is affected by climatic conditions : thus in Brazil 

 Eschscholzia californica is absolutely sterile to the pollen of its own 

 flowers; the descendants of Brazilian plants in Darwin's cultures 

 were partially self-fertile in one generation and in a second genera- 

 tion still more so. If one has any doubt in this case whether it is 

 a question of the condition of the style and stigma, which possibly 

 prevents the entrance of the pollen-tube or even its development, 

 rather than that of the actual sexual cells, in other cases there 

 is no doubt that an influence is exerted on the latter. 



Janczewski^ has recently shown that species of Ribes cultivated 

 under unnatural conditions frequently produce a mixed (i.e. partly 

 useless) or completely sterile pollen, precisely as happens with 

 hybrids. There are, therefore, substantial reasons for the conclusion 

 that conditions of life exert an influence on the sexual cells. " Thus 

 the proposition that the benefit from cross-fertilisation depends on 

 the plants which are crossed having been subjected during previous 

 generations to somewhat diflerent conditions, or to their having 

 varied from some unknown cause as if they had been thus sub- 

 jected, is securely fortified on all sides ^." 



We thus obtain an insight into the significance of sexuality. If an 

 occasional and slight alteration in the conditions under which plants 

 and animals live is beneficial^, crossing between organisms which 

 have been exposed to diflerent conditions becomes still more ad- 

 vantageous. The entire constitution is in this way influenced from 

 the beginning at a time when tlie whole organisation is in a highly 

 plastic state. The total life-energy, so to speak, is increased, a gain 

 which is not produced by asexual reproduction or by the union of 

 sexual cells of plants which have lived under the same or only 

 slightly diflerent conditions. All the wonderful arrangements for 



1 Janczewski, **Sur lea anthSres sterilea des Groaeilliera," Bull, de I'acad. de» science$ 

 de Cracovie, June, 1908. 



» Crott and Self fertiliiation (Ist edit.), p. 444. 



» Reasons for this are given by Darwin in Variation under Domestication (2nd edit.), 

 Vol. II. p. 127. 



D. 27 



