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CHAPTER VIII. 



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 cross-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 measure of the strength 

 of a selective agency perpetually sifting out every modification 

 in the structure of flowers which can affect its capabilities 

 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 off- 

 spring, 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 



VOL. HI. U 



