86 MR. C. DARWIN ON THE DIMORPHIC CONDITION 
and in falling off do not brush over the lowly seated stigma. In 
the Cowslip the corolla does not fall off; and both long-styled and 
short-styled plants are equally sterile when protected from insects. 
It is a rather curious case, that the falling of the corolla, or its re- 
maining attached when withered, might have a considerable in- 
fluence on the numbers of a plant, during a year unfavourable to 
the visits of the proper insects. 
In three short-styled plants of Primula auricula, protected from 
insects, the flowers which I fertilized produced seed, but those 
which were not touched produced none. 
In all the species of Primula the pollen readily coheres to any 
object. In all that I have observed, though the stamens and pis- 
tils differ in length relatively to each other in the different species, 
yet, in the two forms of the same species, the stigma of the one 
form stands at exactly the same height with respect to the corolla 
as the anthers of the other form. If the proboscis of a dead 
Humble-bee, or thick bristle, or rough needle be pushed down the 
corolla, first of one form, and then of the other, as an insect would 
do in visiting the two mingled forms, it will be found that pollen 
from the long-stamened form will adhere round the base of the 
proboscis, and will be left with certainty on the stigma of the long- 
styled form; pollen from the short stamens of the long-styled 
form will also adhere a little above the tip of the proboscis, and 
some will generally be left on the stigma of the other form. Thus 
pollen will be carried reciprocally from one form to the other. In 
withdrawing the proboscis from the long-styled form, with pollen 
adhering near the tip, there will be a good chance of some being 
left on the flower’s own stigma, in which case there will be self- 
fertilization ; but this by no means always occurs. In the short- 
styled form, on the other hand. (and it is important to remember 
this), in inserting the proboscis between the anthers situated at 
the mouth of the corolla, pollen, as I repeatedly found, is almost 
invariably carried down and left on the flower’s own stigma. 
Moreover minute insects, such as Thrips, numbers of which I have 
observed in Primrose flowers thickly dusted with pollen, could not 
fail often to cause self-fertilization. We positively know that the 
visits of large insects are necessary to the fertilization of the species 
of Primula; and we may infer from the facts just given that these 
visits would carry pollen reciprocally from one form to the other, 
and would likewise tend to cause self-fertilization, more especially 
in the short-styled (7. e. long-stamened) form. 
These observations led me to test the potency of the two pol- 
