SELF-FERTILISATION OF PLANTS 107 



mixis is an essential factor in effecting readaptation ; for it dis- 

 seminates the variations which are most adaptive. Thus the 

 retention of amphimixis by natural selection is justified ; for 

 amphimixis is a factor, and, we must conclude from its wide- 

 spread occurrence, a highly important factor, in the conservation 

 of species. In a word, as constancy can only be secured through 

 adaptation and readaptation, we may say of amphimixis that it 

 is the biological instrument of adaptation. 



Note on the Sele-Fertilisation of Plants. 



It is remarkable that among plants a seemingly contradictory pheno- 

 menon is observable. On the one hand, the comitless structural adapta- 

 tions which favour crossiag seem to show that crossing, if not indispensable, 

 is at aU events advantageous ; on the other hand, a small number of plants 

 reproduce exclusively by self -fertilisation, and show no sign of degeneracy. 

 On the one hand, again, we find a number of plants capable of being 

 fertilised by their own poUen, and others on whom the action of their 

 own pollen produces decay or sterility. 



It is probable that this apparent contradiction is due, as Darwin says, 

 to the nature of the sexual elements, and not to any difference in their 

 structure or general constitution ; or, as Weismann puts it, it is a pheno- 

 menon of adaptation. In other words, the fecundity or sterility of a 

 plant after fertilisation by its own poUen will depend upon the degree in 

 which the stigma and the poUen are adapted to harmonious co-operation. 

 Darwin has observed that several causes may contribute to the sterility 

 of first crosses, and several of these causes must be operative in the case 

 of self -fertilisation in plants. For instance, "there must sometimes be 

 a physical impossibility in the male element reaching the ovule, as would 

 be the case with a plant having a pistil too long for the pollen-tubes to 

 reach the ovary. . . . Or, again, the male element may reach the female 

 element, but be incapable of causing an embryo to be developed." In 

 what particular way the constitution of a plant is so adapted that fer- 

 tilisation by its own poUen is impossible, is a question which must in 

 each case be left for botanists to answer. 



The case of those plants which reproduce exclusively by self-fertilisa- 

 tion should suffice to refute the theory that amphimixis is a " rejuvenat- 

 ing " factor, indispensable to the continuance of life, and not merely an 

 advantageous adaptation. But the fact that the self -fertilisation of plants 

 produces satisfactory results cannot be taken as an argument in favour 



