29O THE 'EFFECTS OF CROSS- [l866? 



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 advantage which could not follow if reproductions 

 were entirely asexual. 



It is remarkable that this book, the result of eleven years 

 of experimental work, owed its origin to a chance observation. 

 My father had raised two beds of Linaria vulgaris — one set 

 being the offspring of cross- and the other of self-fertilisation. 

 These plants were grown for the sake of some observations 

 on inheritance, and not with any view to cross-breeding, and he 

 was astonished to observe that the offspring of self-fertilisa- 

 tion were clearly less vigorous than the others. It seemed 

 incredible to him that this result could be due to a single act 

 of self-fertilisation, and it was only in the following year, 

 when precisely the same result occurred in the case of a 

 similar experiment on inheritance in Carnations, that his 

 attention was " thoroughly aroused," and that he determined 

 to make a series of experiments specially directed to the 

 question. The following letters give some account of the 

 work in question :] 



C. Darwin to Asa Gray. 



September 10, [1866?] 



.... I have just begun a large course of experiments on 

 the germination of the seed, and on the growth of the young 

 plants when raised from a pistil fertilised by pollen from the 

 same flower, and from pollen from a distinct plant of the 

 same, or of some other variety. I have not made sufficient 

 experiments to judge certainly, but in some cases the differ- 

 ence in the growth of the young plants is highly remarkable. 

 I have taken every kind of precaution in getting seed from the 

 same plant, in germinating the seed on my own chimney- 

 piece, in planting the seedlings in the same flower-pot, and 

 under this similar treatment I have seen the young seedlings 



