That insects should sometimes fail to cross-fertilise the flowers is intelligible, for I have thrice seen humble-bees of two kinds, as well as hive-bees, sucking the nectar, and they did not depress the keel-petals so as to expose the anthers and stigma; they were therefore quite inefficient for fertilising the flowers. One of these bees, namely, Bombus lapidarius, stood on one side at the base of the standard and inserted its proboscis beneath the single separate stamen, as I afterwards ascertained by opening the flower and finding this stamen prised up. Bees are forced to act in this manner from the slit in the staminal tube being closely covered by the broad membranous margin of the single stamen, and from the tube not being perforated by nectar-passages. On the other hand, in the three British species of Lathyrus which I have examined, and in the allied genus Vicia, two nectar-passages are present. Therefore British bees might well be puzzled how to act in the case of the sweet-pea. I may add that the staminal tube of another exotic species, Lathyrus grandiflorus, is not perforated by nectar-passages, and this species has rarely set any pods in my garden, unless the wing-petals were moved up and down, in the same manner as bees ought to do; and then pods were generally formed, but from some cause often dropped off afterwards. One of my sons caught an elephant sphinx-moth whilst visiting the flowers of the sweet-pea, but this insect would not depress the wing-petals and keel. On the other hand, I have seen on one occasion hive-bees, and two or three occasions the Megachile willughbiella in the act of depressing the keel; and these bees had the under sides of their bodies thickly covered with pollen, and could not thus fail to carry pollen from one flower to the stigma of another. Why then do not the varieties occasionally intercross, though this would not often happen, as insects so rarely act in an efficient manner? The fact cannot, as it appears, be explained by the flowers being self-fertilised at a very early age; for although nectar is sometimes secreted and pollen adheres to the viscid stigma before the flowers are fully expanded, yet in five young flowers which were examined by me the pollen-tubes were not exserted. Whatever the cause may be, we may conclude, that in England the varieties never or very rarely intercross. But it does not follow from this, that they would not be cross by the aid of other and larger insects in their native country, which in botanical works is said to be the south of Europe and the East Indies. Accordingly I wrote to Professor Delpino, in Florence, and he informs me "that it is the fixed opinion of gardeners there that the varieties do intercross, and that they cannot be preserved pure unless they are sown separately."
It follows also from the foregoing facts that the several varieties of the sweet-pea must have propagated themselves in England by self-fertilisation for very many generations, since the time when each new variety first appeared. From the analogy of the plants of Mimulus and Ipomoea, which had been self-fertilised for several generations, and from trials previously made with the common pea, which is in nearly the same state as the sweet-pea, it appeared to me very improbable that a cross between the individuals of the same variety would benefit the offspring. A cross of this kind was therefore not tried, which I now regret. But some flowers of the Painted Lady, castrated at an early age, were fertilised with pollen from the Purple sweet-pea; and it should be remembered that these varieties differ in nothing except in the colour of their flowers. The cross was manifestly effectual (though only two seeds were obtained), as was shown by the two seedlings, when they flowered, closely resembling their father, the Purple pea, excepting that they were a little lighter coloured, with their keels slightly streaked with pale purple. Seeds from flowers spontaneously self-fertilised under a net were at the same time saved from the same mother-plant, the Painted Lady.