92 MR. C. DARWIN ON 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 tbe 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 will 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 effect 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 homomorphie 
pollen when left on the flower's own stigma by insects; and thus 
we see how potent 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 be 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- 
hand figures of the four first lines in the previous tables of P. Si- 
nensis and veris, we shall see that one of the homomorphie 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 unions. 
