214 THE STRUCTURE OF FLOWERS. 



as to reader them permanently dimorphic for legitimate 

 fertilisation. The predominant insect or insects were (as I 

 surmise) the direct cause of arresting the fluctuations which 

 they themselves, as well as accidental sources of nutriment, 

 had set up in the lengths of the essential organs, thus 

 compelling them to retain their anthers and stigmas at the 

 correct height. 



If there were from one to three prominent kinds of 

 insect-visitors the flowers might become adapted to them, 

 and trimorphism be the result ; if four, tetramorphism ; and 

 there is no a priori reason why there should not be polymor- 

 phic flowers as well, in the strict sense of the prefix of that 

 tei'm, provided a flower could furnish a sufficiency of stamens. 



It is further to be noticed that the rule holds good with 

 heterostyled plants, as with all other kinds of differentiation, 

 that in nature, whenever self-fertilisation can be effected, 

 more seed is borne than by the forms requiring intercrossing. 

 First, whenever it can be brought about mechanically ; as 

 has been observed in P. Sinensis, by the corolla, when falling 

 off, dragging the anthers over the stigma in the long-styled 

 form, which consequently yields more seed.* In P. veris, it 

 does not do so ; but as pollen can fall in the short-styled 

 form, in this species that form is thus the most fertile (see 

 above, p. 205). 



Secondly, when these plants are artificially and legiti- 

 mately fertilised, and not left to the chance visits of 

 capricious insects, then the results are as they should be ; 

 but if self-fertilisation be artificially and repeatedly practised, 

 then nature responds to the act ; the anthers and pollen may 

 in part degenerate, but what is left good is ample to secure 

 abundant seed, and the self-fertilised form surpasses even the 



* Darwin found that, in the absence of insects, the long-styled form 

 of P. Sinensis was twenty-four times as productive as the short-styled. 



