CROSS-FERTILIZATION AND ASYNGAMY 91 



fully close parallelism exists between hybridisation and 

 certain forms of fertilisation among heterostyled plants. 

 So that it is hardly an exaggeration to say that the 

 " illegitimately " reared seedlings are hybrids, although 

 both their parents belong to identically the same species. 

 In a letter to Professor Huxley, given in the second 

 volume [of Life and Letters] (p. 384), my father writes 

 as if his researches on heterostyled plants tended to make 

 him believe that sterility is a selected or acquired quality. 

 But in his later publications, e. g. in the sixth edition 

 of the " Origin ", he adheres to the belief that sterility 

 is an incidental rather than a selected quality. The 

 result of his work on heterostyled plants is of importance 

 as showing that sterility is no test of specific distinctness, 

 and that it depends on differentiation of the sexual 

 elements which is independent of any racial difference.' 1 



The different forms of a heterostyled plant are adapted 

 for cross-fertilization by insects, and each individual of 

 each form is by the same means excluded more or less 

 completely from fertilization by another of the same form. 

 In the former case the sexual cells and the accessory 

 apparatus have been kept by selection during long genera- 

 tions of Syngamy in a high state of mutual compatibility ; 

 in the latter Asyngamy, partial or complete, has produced 

 a large measure of the sterility which is its inevitable 

 even if long-delayed result. 



Are the injurious effects of Self-Fertilization the conse- 



qiience and not the cause of the Adaptations 



for Cross- Fertilization t 



The argument based upon heterostyled plants has, 

 I admit, carried me much further than I originally in- 

 tended, and it will be a pleasure to me if the following 

 criticism can be overthrown. 



If the special adaptation of heterostyled plants for 

 particular lines of Syngamy has incidentally resulted 

 in lessened fertility, when the unions discouraged by 

 these adaptations are artificially secured, and in this case 



1 Life and Letters, vol. iii, p. 296. 



