92 ilE. C. DAKWIN OK THE DIMORPHIC CONDITION 



forms in about equal numbers, with their pollen adapted for reci- 

 procal union, is tolerably plain ; namely, to favour the intercross- 

 ing of distinct individuals. With plants there are innumerable 

 contrivances for this end ; and no one will understand the final 

 cause of the structure of many flowers without attending to this 

 point. I have already shown that the relative heights of the an- 

 thers and stigmas in the two forms lead to insects leaving the 

 pollen of the one form on the stigma of the other ; but, at the same 

 time, there will be a strong probability of the flower's own pollen 

 being likewise placed on the stigma. It is perfectly well known 

 that if the pollen of several closely allied species be placed on the 

 stigma of a distinct species, and at the same time, or even subse- 

 quently, its own pollen be placed on the stigma, this wUl entirely 

 destroy the simultaneous or previous action of the foreign pollen. 

 So again if the pollen of several varieties, including the plant's own 

 pollen, be placed on the stigma, one or more of the varieties will 

 take the lead and obliterate the efiiect of the others : but I have 

 not space here to give the facts on which this conclusion is 

 grounded. Hence we may infer as highly probable that, in Pri- 

 mula, the heteromorphic pollen which we know to be so much the 

 most effective would obliterate the action of the homomorphic 

 pollen when left on the flower's own stigma by insects ; and thus 

 we sec how poteut the dimorphic condition of the pollen in Pri- 

 mula will be in favouring the intercrossing of distinct individuals. 

 The two forms, though both sexes are present in each, are in 

 fact dioicous or unisexual. Whatever advantage there may bo in 

 the separation of the sexes, towards which we see so frequent a 

 tendency throughout nature, this advantage has been here so far 

 gained, that the one form is fertilized by the other, and conversely ; 

 and this is effected by the pollen of each form having less po- 

 tency than that of the other on its own stigma. 



Bearing on this view of the final cause of the dimorphism of the 

 Primulas, there is another curious point. If we look at the right- 

 haud figures of the four first lines in the previous tables of P. Si- 

 nensis and veris, we shall see that one of the homomorphic unions, 

 namely, the short-styled by its own-form pollen, is considerably 

 more sterile than the other ; and in P. auricula, though here there 

 is no other homomorphic union as a standard of comparison, this 

 union is likewise excessively sterile. That the fertility of this 

 union is really less in a marked degree than in the other three 

 unions, we have an independent proof in the seeds germinating less 

 perfectly and much more slowly than those from the other imions. 



