CHAP. IV. LYTHEUM SALICAEIA. 161 



legitimate fertilisation. But the rule of the increased 

 sterility of the illegitimate unions according to the 

 greater inequality in length between the pistils and 

 stamens employed for the union can be of no service. 

 With some heterostyled dimorphic plants the dif- 

 ference of fertility between the two illegitimate unions 

 appears at first sight to be related to the facility of 

 self-fertilisation ; so that when from the position of 

 the parts the liability in one form to self-fertilisation 

 is greater than in the other, a union of this kind 

 has been checked by having been rendered the 

 more sterile of the two. But this explanation does 

 not apply to Lythrum ; thus the stigma of the long- 

 styled form is more liable to be illegitimately fer- 

 tilised with pollen from its own mid-length stamens, 

 or with pollen from the mid-length stamens of the 

 short-styled form, than by its own shortest stamens 

 or those of the mid-styled form ; yet the two former 

 unions, which it might have been expected would 

 have been guarded against by increased sterility, 

 are much less sterile than the other two unions 

 which are much less likely to be effected. The 

 same relation holds good even in a more striking 

 manner with the mid-styled form, and with the short- 

 styled form as far as the extreme sterility of all its 

 illegitimate unions allows of any comparison. We 

 are led, therefore, to conclude that the rule of in- 

 creased sterility in accordance with increased in- 

 equality in length between the pistils and stamens, 

 is a purposeless result, incidental on those changes 

 through which the species has passed in acquiring 

 certain characters fitted to ensure the legitimate 

 fertilisation of the three forms. 



Another conclusion which may be drawn from 

 Tables 23, 24, and 25, even from a glance at them, 



