CHAPTER XI. 



THE * EFFECTS OF CROSS- AND SELF-FERTILISATION IN THE 



VEGETABLE KINGDOM.' 1876. 



[This book, as pointed out in the 'Autobiography,' is a 

 complement to the * Fertilisation of Orchids,' because it shows 

 how important are the results of cross-fertilisation which are 

 ensured by the mechanisms described in that book. By 

 proving that the offspring of cro'ss-fertilisation are more 

 vigorous than the offspring of self-fertilisation, he showed that 

 one circumstance which influences the fate of young plants in 

 the struggle for life is the degree to which their parents are 

 fitted for cross-fertilisation. He thus convinced himself that 

 the intensity of the struggle (which he had elsewhere shown 

 to exist among young plants) is a m^easure of the strength 

 of a selective agency perpetually sifting out every modifica- 

 tion in the structure of flowers which can effect its capabili- 

 ties for cross-fertilisation. 



The book is also valuable in another respect, because it 

 throws light on the difficult problems of the origin of sexuality. 

 The increased vigour resulting from cross-fertilisation is allied 

 in the closest manner to the advantage gained by change of 

 conditions. So strongly is this the case, that in some instances 

 cross-fertilisation gives no advantage to the offspring, unless 

 the parents have lived under slightly different conditions. 

 So that the really important thing is not that two individuals 

 of different blood shall unite, but two individuals which have 

 been subjected to different conditions. We are thus led to 

 believe that sexuality is a means for infusing vigour into the 

 offspring by the coalescence of differentiated elements, an 



