Cha?. XII. GENERAL RESULTS. 463 



among plants which have been self-fertilised and 

 grown under uniform conditions during several gene- 

 rations. We can form no conception why the 

 advantage from a cross is sometimes directed exclu- 

 sively to the vegetative system, and sometimes to the 

 reproductive system, but commonly to both. It is 

 equally inconceivable why some individuals of the same 

 species should be sterile, whilst others are fully fertile 

 with their own pollen ; why a change of climate should 

 either lessen or increase the sterility of self-sterile 

 species; and why the individuals of some species should 

 be even more fertile with pollen from a distinct species 

 than with their own pollen. And so it is with many 

 other facts, which are so obscure that we atand in 

 awe before the mystery of life. 



Under a practical point of view, agriculturists and 

 horticulturists may learn something from the conclu- 

 sions at which we have arrived. Firstly, we see that 

 the injury from the close breeding of animals and 

 from the self-fertilisation of plants, does not necessarily 

 depend on any tendency to disease or weakness of con- 

 stitution common to the related parents, and only 

 indirectly on their relationship, in so far as they are 

 apt to resemble each other in all respects, including 

 their sexual nature. And, secondly, that the advantages 

 of cross-fertilisation depend on the sexual elements of 

 the parents having become in some degree differentiated 

 by the exposure of their progenitors to different 

 conditions, or from their having intercrossed with 

 individuals thus exposed, or, lastly, from what we call 

 in our ignorance spontaneous variation. He therefore 

 who wishes to pair closely related animals ought to 

 keep them under conditions as different as possible. 

 Some few breeders, guided by their keen powers of 



