Observations on the Primrose and the Cowslip. 169 
as the wind or some minute insects like thrips which are very 
commonly met with in the flowers in some localities. In the 
absence of these agencies these plants remain sterile, as I have 
clearly proved this spring with plants of both the short and long 
styled forms grown under glass. That the glass itself did not in any 
way interfere with the fertility of the plants, as Mr. Bell supposed 
the nets used by Darwin did, is shown by the fact that control- 
plants, self and cross-pollinated by artificial means matured their 
seeds readily. The fact of th,e matter is that the truth lies between 
the two extreme views, the one put forward by Mr. Bell who dis¬ 
believed in the natural cross-fertilisation of the Primulaceae and the 
other extreme maintained by Lord Avebury who has stated that 
heterostyly is “ one of the principal modes by which self-fertilisation 
is prevented.” (“ Flowersand Insects.”) A microscopic examination 
of the stigma of a large number of primrose flowers from very 
different localities has shown that a considerable portion have 
pollen-grains from both the long-styled and the short-styled flowers 
fixed among their hairs, and there is no difficulty in distinguishing 
the two kinds of pollen. The proportion of the large and small pollen- 
grains varied very much and as the flowers were not examined on 
the spot it is impossible to make any deductions from the relative 
number of the two kinds of grains. As a rule there were on the 
stigma more pollen-grains similar to those produced in the flower 
itself, but the presence of even a few pollen-grains of a larger or 
smaller size shows that the visits of the insects have been effective 
in producing cross-pollination, and if, as Darwin showed, this pollen 
possessed any prepotency, the number of cross fertilized ovules might 
be a quite considerable proportion of the total seeds produced. 
With regard to the insects I had observed last year I was 
anxious to find out whether the same insects were the fertilising 
agents in other localities, especially as Mr. Bell in a private letter 
to me had expressed a doubt whether Bombylius, which I had found 
most frequently on the primroses at Church Stretton, was common 
enough to account for the cross-pollination of so widespread a 
flower as the primrose. I therefore visited another locality this 
spring. Unfortunately the Easter Vacation was rather too early 
this year to see the primroses at their best, and I had some difficulty 
in North Staffordshire, where I was staying, to find a sheltered bank 
where the primroses were plentiful and sufficiently forward. But 
on two mornings out of the three on which 1 visited a favourable 
spot I found the bee-fly ( Bombylius ) busy on the flowers of the 
