264 CONCLUDING KEMABKS Chap. VI, 



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 originated. 

 A completely self -sterile plant or a dichogamous 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 can- 

 not fully fertilise or be fertilised 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 other 

 manner will be nearly or completely lost. He was led 

 to this view by observing that Diptera frequently car- 

 ried pollen from the long-styled flowers of Hottonia to 

 the stigma of the same form, and that this illegitimate 

 union was not nearly so sterile as the corresponding 

 union in other heterostyled species. But this conclu- 

 sion is directly opposed by some other cases, for instance 

 by that of Linum grandiflorum; for here the long- 

 styled form is utterly barren with its own-form pollen, 

 although from the position of the anthers this pollen 



• ' Die Befruchtung der Blumen,' p. 352. 



