Abstract

THE extension of our perceptive faculties of sight and hearing by various optical and acoustical instruments may enable us to comprehend the possibility of these faculties existing in other creatures to a degree so far surpassing ours as to seem a difference almost of kind. So the sight of the vulture would seem to be paralleled by the faculty of smell in moths, as evidenced by the detection of distant females by males. It would seem probable that the sense of smell may guide insects at a far greater distance than that of vision; for a consideration of the structure of the eyes of insects leads to the belief that they are not capable of forming clear images of distant objects. While, then, the scent of its blossoms may attract insects to a plant, their colour will act as a subsequent guide to the individual flowers, just as variegations undoubtedly act as honey-guides when the insect reaches the flower. This view is borne out, firstly, by the undoubted connection between perfume and pollination, shown by Morren in the case of the orchid Maxillaria, whose aromatic perfume lasts till pollination; and, secondly, by the well known connection of odour both with colour and with natural groups, white flowers being mostly sweet-scented, brown and orange ones most fetid. The insect could thus identify species before seeing them. Mr. Wallace has been, perhaps justly, blamed by a writer in the Gardener's Chronicle for saying that brightly-coloured flowers are seldom scented, and Dr. Taylor by “J. S. G”. (NATURE, vol. xviii. p. 277), for saying that white flowers open mostly at night. It would, I think, be truer to say that few flowers are both variegated and scented, i.e., that scented flowers are mostly monochromous, and that the majority of night-blowing flowers are white. The latter is a very different matter from saying that the majority of white flowers are night-blowing. We can perceive with difficulty that one part of a flower is more scented than another, yet scent may replace the dots and point-indicating lines of variegation to the senses of an insect. Nature not only often effects one purpose by divers means, but also uses one means for divers ends; so just as colour exists in plants, not only to attract insects,. we can understand it being absent in some white flowers simply as a phenomenon of degradation and not as one of specialisation. The dog-rose, white convolvulus, and daisy, mentioned by Mr. Gardner as closing at eventide, are all scentless. The first, according to Dr. Hermann Müller, is visited by six hymenoptera, two diptera, and twelve coleoptera. The convolvulus does not close till between eight and ten P.M., and re-opens by moonlight. It is visited by two diptera, Podura, Thrips, one coleopteron, two hymenoptera, and the Sphinx conovolvuli, L. This is a dusk-loving hawk-moth, which also visits the honeysuckle. The daisy is visited by nine hymenoptera, thirteen diptera, three coleoptera, and two lepicloptera, viz., the least meadow brown and the common blue butterflies. Many flowers, like Lychnis vespertina, remain open without exhaling their perfume, and I think Mr. Gardner will find that most of the subduedly-coloured flowers which are open at night give off most perfume, and are visited and fertilised by moths rather at dusk than in the dark, whilst the white ones remain fragrant still later. The clearly-cut discs of white of Lychnis vespertina are the last objects our eyes can often discern on a midsummer night's ramble. Of course variegation on the moths themselves would be as-useless, from-the point of view of sexual selection, as on the flowers from that of insect-fertilisation. Though it is to a certain extent true that like causes produce like effects, in investigations into phenomena so complex in their etiology as those of biology we must, I think, be more mindful that the converse that like effects are the result of like causes by no means necessarily follows.