Fagopyrum is dimorphic, and Anagallis collina is non-dimorphic, and the crossed seedlings of both are in height to the self-fertilised as 100 to 69.
With all European plants, excepting the comparatively rare anemophilous kinds, the possibility of distinct individuals intercrossing depends on the visits of insects; and Hermann Muller has proved by his valuable observations, that large conspicuous flowers are visited much more frequently and by many more kinds of insects, than are small inconspicuous flowers. He further remarks that the flowers which are rarely visited must be capable of self-fertilisation, otherwise they would quickly become extinct. (10/25. 'Die Befruchtung' etc. page 426. 'Nature' 1873 page 433.) There is, however, some liability to error in forming a judgment on this head, from the extreme difficulty of ascertaining whether flowers which are rarely or never visited during the day (as in the above given case of Fumaria capreolata) are not visited by small nocturnal Lepidoptera, which are known to be strongly attracted by sugar. (10/26. In answer to a question by me, the editor of an entomological journal writes--"The Depressariae, as is notorious to every collector of Noctuae, come very freely to sugar, and no doubt naturally visit flowers:" the 'Entomologists' Weekly Intelligencer' 1860 page 103.) The two lists given in the early part of this chapter support Muller's conclusion that small and inconspicuous flowers are completely self-fertile: for only eight or nine out of the 125 species in the two lists come under this head, and all of these were proved to be highly fertile when insects were excluded. The singularly inconspicuous flowers of the Fly Ophrys (O. muscifera), as I have elsewhere shown, are rarely visited by insects; and it is a strange instance of imperfection, in contradiction to the above rule, that these flowers are not self-fertile, so that a large proportion of them do not produce seeds. The converse of the rule that plants bearing small and inconspicuous flowers are self-fertile, namely, that plants with large and conspicuous flowers are self-sterile, is far from true, as may be seen in our second list of spontaneously self-fertile species; for this list includes such species as Ipomoea purpurea, Adonis aestivalis, Verbascum thapsus, Pisum sativum, Lathyrus odoratus, some species of Papaver and of Nymphaea, and others.
The rarity of the visits of insects to small flowers, does not depend altogether on their inconspicuousness, but likewise on the absence of some sufficient attraction; for the flowers of Trifolium arvense are extremely small, yet are incessantly visited by hive and humble-bees, as are the small and dingy flowers of the asparagus. The flowers of Linaria cymbalaria are small and not very conspicuous, yet at the proper time they are freely visited by hive-bees. I may add that, according to Mr. Bennett, there is another and quite distinct class of plants which cannot be much frequented by insects, as they flower either exclusively or often during the winter, and these seem adapted for self-fertilisation, as they shed their pollen before the flowers expand. (10/27. 'Nature' 1869 page 11.)
That many flowers have been rendered conspicuous for the sake of guiding insects to them is highly probable or almost certain; but it may be asked, have other flowers been rendered inconspicuous so that they may not be frequently visited, or have they merely retained a former and primitive condition? If a plant were much reduced in size, so probably would be the flowers through correlated growth, and this may possibly account for some cases; but the size and colour of the corolla are both extremely variable characters, and it can hardly be doubted that if large and brightly-coloured flowers were advantageous to any species, these could be acquired through natural selection within a moderate lapse of time, as indeed we see with most alpine plants. Papilionaceous flowers are manifestly constructed in relation to the visits of insects, and it seems improbable, from the usual character of the group, that the progenitors of the genera Vicia and Trifolium produced such minute and unattractive flowers as those of V.