84 MR. C. DARWIN ON THE DIMORPHIC CONDITION 
than the long-styled forms ; consequently the anticipation that the 
plants having largely developed pistils with rougher stigmas, and 
having shorter stamens with smaller pollen-grains, would prove to 
be more feminine in their nature is exactly the reverse of the 
truth. Ifthe species of Primula are tending to become dioicous, 
whieh possibly may be the case, the future hypothetical females 
would have short pistils, and the males would have short stamens ; 
but this tendency is accompanied, as we shall presently see, by 
other conditions of the generative system of a much more singular 
nature. Anyhow, the possibility of a plant thus becoming dioicous 
by slow degrees is worthy of notice, as the fact would so easily 
escape observation. 
In 1860 I found that a few umbels of both long-styled and 
short-styled Cowslips, which were covered by a net, did not pro- 
duce seed, though other umbels on the same plants, artificially fer- 
tilized, produced an abundance of seed; and this fact shows that 
the mere covering in itself was not injurious. Accordingly, in 
1861 I covered up under a similar net several plants just before 
they opened their flowers; these turned out as follows :— 
" No. of 
Def | umbels Product of Seed. — | 
produced, | 
Short-styled ............... 6 24 1:3 grains, or 50 seeds. 
| Long-styled ...... ......... 18 74 | Not one seed. 
Judging from the exposed plants which grew all round in the 
same bed, and bad been treated in every way exactly the same, 
except that they were exposed to the visits of insects, the six short- 
styled plants ought to have produced 92 grains’ weight of seed in- 
stead of only 1:3; and the eighteen long-styled plants, which pro- 
duced not one seed, ought to have produced above 200 grains’ 
weight. The production of the 1:3 grain of seed in the smaller 
lot was probably due to the action of Thrips or some minute in- 
sect. This evidence is sufficient, but I may add that ten pots of 
Polyanthuses and Cowslips of both forms, protected from insects 
in my greenhouse, did not set one pod, though artificially fertilized 
flowers in other pots produced an abundance. So we see that the 
visits of insects are absolutely necessary to the fertilization of 
Cowslips. As the exposed plants produced an abundance of seed, 
the tendency to a dioicous condition, previously remarked on, 
might have been safely carried on, as we see that there is an effect- 
