KNIGHT 157 



of crossing the breeds, for the smallest variety, whose 

 height rarely exceeded two feet, was increased to six 

 feet, whilst the height of the large and luxuriant kind 

 was very little diminished.' 



We shall see, however, that the phenomenon last 

 alluded to admits of a different interpretation. 



It was upon the somewhat slender basis afforded 

 by this experiment that the generalization known as 

 the Knight-Darwin law was originally established. 

 Knight's own expression of this idea was to the effect 

 that ' Nature intended that a sexual intercourse should 

 take place between neighbouring plants of the same 

 species.' And the same conclusion was expressed still 

 more forcibly by Darwin in the aphorism : ' Nature 

 abhors perpetual self-fertilization.' But although it 

 may be true that in a considerable number of cases 

 advantages are gained from the process of cross- 

 fertilization between different members of the same 

 species, which do not accrue when self-fertilization 

 takes place, yet several cases are now known in which 

 self-fertilization really does seem to be indefinitely 

 continued. 



Knight crossed a pea having white flowers and seed- 

 coats, and green stems, with one in which the flowers 

 and stems were coloured purple, and the seeds grey. 

 The seeds immediately resulting from the cross were 

 unchanged in appearance, but the plants arising from 

 these took closely after their coloured male parent. 

 On crossing the cross-bred plants once more with a 

 white strain a certain proportion of white plants was 

 again obtained, though what that proportion was 



