CHAP. VI. ON HETEROSTYLED PLANTS. 263 



viduals in which the stamens were greatly developed, 

 and to be increased in length in those which had their 

 stamens but little developed. 



Now if in our varying species the longer stamens 

 were to be nearly equalised in length in a considerable 

 body of individuals, with the pistil more or less reduced ; 

 and in another body, the shorter stamens to be simi- 

 larly equalised, with the pistil more or less increased in 

 length, cross-fertilisation would be secured with little 

 loss of pollen ; and this change would be so highly 

 beneficial to the species, that there is no difficulty in be- 

 lieving that it could be effected through natural selec- 

 tion. Our plant would then make a close approach in 

 structure to a heterostyled dimorphic species ; or to a 

 trimorphic species, if the stamens were reduced to two 

 lengths in the same flower in correspondence with that 

 of the pistils in the other two forms. But we have not 

 as yet even touched on the chief difficulty in under- 

 standing how heterostyled species could have origi- 

 nated. A completely self-sterile plant or a dicho- 

 gamous one can fertilise and be fertilised by any 

 other individual of the same species; whereas the 

 essential character of a heterostyled plant is that an 

 individual of one form cannot fully fertilise or be fer- 

 tilised by an individual of the same form, but only 

 by one belonging to another form. 



H. Miiller has suggested * that ordinary or homo- 

 styled plants may have been rendered heterostyled 

 merely through the effects of habit. Whenever pollen 

 from one set of anthers is habitually applied to a pistil 

 of particular length in a varying species, he believes 

 that at last the possibility of fertilisation in any othox 



' Die Befrachtung der Blumen,' p. 352. 



18 



