92 'WHAT IS A SPECIES?' 



without appeal to the physiologically injurious effects 

 of self-fertilization, why should we not similarly explain 

 these effects whenever manifest in the self-bred 1 offspring 

 of any plant especially adapted for cross-fertilization ? 



Darwin tells us in the Autobiography that as soon as 

 his 'attention was thoroughly aroused to the remarkable 

 fact that seedlings of self-fertilised parentage are inferior, 

 even in the first generation, in height and vigour to 

 seedlings of cross-fertilised parentage ', 2 he entered upon 

 a series of experiments which lasted eleven years, ap- 

 pearing in 1876 as Effects of Cross and Self -Fertilisa- 

 tion in the Vegetable Kingdom. Of this work he wrote 

 in 1 88 1, ' the results there arrived at explain, as I believe, 

 the endless and wonderful contrivances for the transportal 

 of pollen from one plant to another of the same species.' " 

 It is here suggested that these injurious results have 

 been not the cause but the consequence of specialization 

 for cross-fertilization. In such plants fertilization is 

 mainly brought about along the line for which special 

 adaptation is made : self-fertilization is relatively infre- 

 quent, often very rare, sometimes perhaps absent alto- 

 gether. May not the less successful results have followed 

 from a condition in which self-fertilization is but little 

 tried by the fires of selection? 4 It would be of much 

 interest to compare a long series of experiments on 

 the cross-fertilization of plants which are habitually 

 self-fertilized and on the self-fertilization of plants in 

 which the adaptations for cross-fertilization are made 

 use of in widely different degrees. 



This criticism, should it be sustained, would of course 

 throw much light upon the case of the Bee Orchis and the 

 numbers of tropical Orchidaceae, &c., which are now 

 known to be regularly self-fertilizing without apparent 

 physiological injury. It would also bear powerfully upon 

 an intrusive set of facts which must often have weighed 

 upon the minds of naturalists as they reflected upon the 



1 See The Knight-Darwin Law, by Francis Darwin in Nature, Octo- 

 ber 27, 1898, p. 630. 



2 Life and Letters, vol. i, p. 96. 3 Ibid. vol. i, p. 97. 



4 See also A. R. Wallace in Darwinism, London, 1889, pp. 321-6. 



