168 THE OLDER HYBRIDISTS 
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 
