(10/28. Some of the species of Solanum would be good ones for such experiments, for they are said by Hermann Muller 'Befruchtung' page 434, to be unattractive to insects from not secreting nectar, not producing much pollen, and not being very conspicuous. Hence probably it is that, according to Verlot 'Production des Varieties' 1865 page 72, the varieties of "les aubergines et les tomates" (species of Solanum) do not intercross when they are cultivated near together; but it should be remembered that these are not endemic species. On the other hand, the flowers of the common potato (S. tuberosum), though they do not secrete nectar Kurr 'Bedeutung der Nektarien' 1833 page 40, yet cannot be considered as inconspicuous, and they are sometimes visited by diptera (Muller), and, as I have seen, by humble-bees. Tinzmann (as quoted in 'Gardeners' Chronicle' 1846 page 183, found that some of the varieties did not bear seed when fertilised with pollen from the same variety, but were fertile with that from another variety.)
It should be remembered that in two of the cases in which highly self-fertile varieties appeared amongst my experimental plants, namely, with Mimulus and Nicotiana, such varieties were greatly benefited by a cross with a fresh stock or with a slightly different variety; and this likewise was the case with the cultivated varieties of Pisum sativum and Lathyrus odoratus, which have been long propagated by self-fertilisation. Therefore until the contrary is distinctly proved, I must believe that as a general rule small and inconspicuous flowers are occasionally intercrossed by insects; and that after long-continued self-fertilisation, if they are crossed with pollen brought from a plant growing under somewhat different conditions, or descended from one thus growing, their offspring would profit greatly. It cannot be admitted, under our present state of knowledge, that self-fertilisation continued during many successive generations is ever the most beneficial method of reproduction.
THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN FROM A DISTINCT PLANT.
We have seen in four cases that seedlings raised from a cross between flowers on the same plant, even on plants appearing distinct from having been propagated by stolons or cuttings, were not superior to seedlings from self-fertilised flowers; and in a fifth case (Digitalis) superior only in a slight degree. Therefore we might expect that with plants growing in a state of nature a cross between the flowers on distinct individuals, and not merely between the flowers on the same plant, would generally or often be effected by some means. The fact of bees and of some Diptera visiting the flowers of the same species as long as they can, instead of promiscuously visiting various species, favours the intercrossing of distinct plants. On the other hand, insects usually search a large number of flowers on the same plant before they fly to another, and this is opposed to cross-fertilisation. The extraordinary number of flowers which bees are able to search within a very short space of time, as will be shown in a future chapter, increases the chance of cross-fertilisation; as does the fact that they are not able to perceive without entering a flower whether other bees have exhausted the nectar. For instance, Hermann Muller found that four-fifths of the flowers of Lamium album which a humble-bee visited had been already exhausted of their nectar. (10/29. 'Die Befruchtung' etc. page 311.) In order that distinct plants should be intercrossed, it is of course indispensable that two or more individuals should grow near one another; and this is generally the case. Thus A. de Candolle remarks that in ascending a mountain the individuals of the same species do not commonly disappear near its upper limit quite gradually, but rather abruptly. This fact can hardly be explained by the nature of the conditions, as these graduate away in an insensible manner, and it probably depends in large part on vigorous seedlings being produced only as high up the mountain as many individuals can subsist together.