Chap. XII. GENERAL RESULTS. 439 



CHAPTER XII. 



Gexeral Results. 



Cross-fertilisation proved to be beneficial, and self-fertilisation in- 

 jurious Allied species differ greatly in the means by which cross- 

 fertilisation is favoured and self-fertilisation avoided The benefits 

 and evils of the two processes depend on the degree of differentiation 

 in the sexual elements The evil effects not due to the combination of 

 morbid tendencies in the parents Nature of the conditions to which 

 plants are subjected when growing near together in a state of nature 

 or under culture, and the effects of such conditions Theoretical 

 considerations with respect to the interaction of differentiated sexual 

 elements Practical lessons Genesis of the two sexes Close corre- 

 spondence between the effects of cross-fertilisation and self-fertilisa- 

 tion, and of the legitimate and illegitimate unions of heterostyled 

 plants, in comparison with hybrid unions. 



The first and most important of the conclusions which 

 may be drawn from the observations given in this 

 volume, is that generally cross-fertilisation is beneficial, 

 and self-fertilisation often injurious, at least with the 

 plants on which I experimented. Whether long- 

 continued self -fertilisation is injurious to all plants 

 is another and difficult question. The truth of these 

 conclusions is shown by the difference in height, 

 weight, constitutional vigour, and fertility of the 

 offspring from crossed and self-fertilised flowers, and 

 in the number of seeds produced by the parent-plants. 

 With respect to the second of the two propositions, 

 namely, that self-fertilisation is often injurious, we have 

 abundant evidence. The structure of the flowers in 

 such plants as Lobelia ramosa, Digitalis purpurea, &c, 

 renders the aid of insects almost indispensable for 



