With respect to the average number of seeds per capsule hardly anything need be said: supposing that the legitimately fertilised capsules contained, on an average, 50 seeds, and the illegitimately fertilised capsules 25 seeds; then as 50 is to 25 so is 100 to 50; and the latter number would appear in the right hand column.

It is impossible to look at the above table and doubt that the legitimate unions between the two forms of the above nine species of Primula are much more fertile than the illegitimate unions; although in the latter case pollen was always taken from a distinct plant of the same form. There is, however, no close correspondence in the two rows of figures, which give, according to the two standards, the difference of fertility between the legitimate and illegitimate unions. Thus all the flowers of P. Sinensis which were illegitimately fertilised by Hildebrand produced capsules; but these contained only 42 per cent of the number of seeds yielded by the legitimately fertilised capsules. So again, 95 per cent of the illegitimately fertilised flowers of P. Sikkimensis produced capsules; but these contained only 31 per cent of the number of seeds in the legitimate capsules. On the other hand, with P. elatior only 27 per cent of the illegitimately fertilised flowers yielded capsules; but these contained nearly 75 per cent of the legitimate number of seeds. It appears that the setting of the flowers, that is, the production of capsules whether good or bad, is not so much influenced by legitimate and illegitimate fertilisation as is the number of seeds which the capsules contain. For, as may be seen at the bottom of Table 1.12, 88.4 per cent of the illegitimately fertilised flowers yielded capsules; but these contained only 61.8 per cent of seeds, in comparison, in each case, with the legitimately fertilised flowers and capsules of the same species. There is another point which deserves notice, namely, the relative degree of infertility in the several species of the long-styled and short-styled flowers, when both are illegitimately fertilised. The data may be found in the earlier tables, and in those given by Mr. Scott in the Paper already referred to. If we call the number of seeds per capsule produced by the illegitimately fertilised long-styled flowers 100, the seeds from the illegitimately fertilised short- styled flowers will be represented by the following numbers (Table 1.a.):--

TABLE 1.a.

Primula veris : 71.

Primula elatior : 44 (Probably too low).

Primula vulgaris : 36 (Perhaps too low).

Primula Sinensis : 71.

Primula auricula : 119.

Primula Sikkimensis : 57.

Primula cortusoides : 93.

Primula involucrata : 74.

Primula farinosa : 63.

We thus see that, with the exception of P. auricula, the long-styled flowers of all nine species are more fertile than the short-styled flowers, when both forms are illegitimately fertilised. Whether P. auricula really differs from the other species in this respect I can form no opinion, as the result may have been accidental. The degree of self-fertility of a plant depends on two elements, namely, on the stigma receiving its own pollen and on its more or less efficient action when placed there. Now as the anthers of the short-styled flowers of several species of Primula stand directly above the stigma, their pollen is more likely to fall on it, or to be carried down to it by insects, than in the case of the long-styled form. It appears probable, therefore, at first sight, that the lessened capacity of the short-styled flowers to be fertilised with their own pollen, is a special adaptation for counteracting their greater liability to receive their own pollen, and thus for checking self-fertilisation. But from facts with respect to other species hereafter to be given, this view can hardly be admitted. In accordance with the above liability, when some of the species of Primula were allowed to fertilise themselves spontaneously under a net, all insects being excluded, except such minute ones as Thrips, the short-styled flowers, notwithstanding their greater innate self-sterility, yielded more seed than did the long-styled.

Charles Darwin

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