HETEROSTYLISM. 213 



Origin of Heterostylism. — The question maj be now- 

 asked, How has heterostylism arisen ? We have seen, in the 

 first place, that in many cases there is a certain instabilit}- in 

 the length of the filaments of the stamens and of the styles, 

 in that they are liable to alter spontaneously, and especially 

 under cultivation.* In the case of Primula Auricula^ the 

 homomorphic form has the anthers and stigma at the orifice, 

 while in P. Sine?isis they are often both low down ; it is clear 

 that either might arise in two ways. In the case of the former, 

 the stamens, while resembling in position that of the stamens 

 in the short-styled, form, have pollen like that of the long- 

 styled, the pistil being of that kind. Hence it is reasonable 

 to assume that the anthers have been uplifted. In the 

 Chinese Primrose it is the reverse ; so that the pistil 

 of a long-styled form has been lowered to the level of the 

 stamens; the stigmas, too, are that of the short-styled. 



Eecognizing this instability of the essential organs, it is 

 reasonable to assume that it may be due to varying degrees 

 of nutrition which can readily bring about such changes, a 

 relatively strong vegetative vigour elevating the stamens in 

 the one case, while a slight tendency to degeneracy with 

 lessened vital vigour tends to suppress the pistil in the other. 



Assuming a homomorphic form to have been the primitive 

 and ancestral state," we can realize how dimorphism has been 

 brought about by such varying degrees of stimulus having 

 been applied to the stamens and pistil. Insect agency 

 I take to have been this cause, which, at the same time, has 

 by selection Jixed the heights of the stamens and style so 



* See the description, given above, of Narcissus cernuus, Fig. 37, 

 p. 121. Mr. Darwin found Qilia to vary mnch in this respect. It may 

 be added that it is a not uncommon feature in flowers which are not 

 heterostyled, as e.g. cultivated Gladioli and Croci, Fritillaria Meleagris, 

 etc. 



