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Show 92 NATURAL SELECTION. CHAP. IV. effected by glands at the base of the stipules in some Leguminosoo, and at the back of the leaf of the common laurel. This juice, though small in quantity, is greedily sought by insects. Let us now suppose a little sweet juice or nectar to be excreted by the inner bases of the petals of a flower. In this case insects in seeking the nectar would get dusted with pollen, and would certainly often transport the pollen from one flower to the stigma of another flower. The flowers of two distinct individuals of the same species would thus get crossed; and the act of crossing, we have good reason to believe (as will hereafter be more fully alluded to), would produce veryvigorous seedlings, which consequently would have the best chance of flourishing and surviving. Some of these seedlings would probably inherit the nectar-excreting power. Those individual flowers which had the largest glands or nectaries, and which excreted most nectar would be oftenest visited by insects, and would b~ oftenest crossed; and so in the long-run would gain the upper hand. Those flowers, also, which had their stamens and pistils placed, in relation to the size and habits of the particular insects which visited them so as to favour in any degree the transportal of their ~ollen from flower to flower, would likewise be favoured or selected. We might have taken the case of insects visiting flowers for the sake of collecting pollen instead of nec~~r; .and. as pollen i~ formed for the sole object of fert1hsatwn, Its destructiOn appears a simple loss to the P.lant ; yet if a little pollen were carried, at first ~ccas~onally and then habitually, by the pollen-devourIng Insects from flower to flower, and a cross thus effected, . alt~ough nine-tenths of the pollen were destroye~, l~ ~xnght still ·be a great gain to the plant ; and those Individuals which produced more and more pollen and had larger and larger anthers, would be selected. ' .CHAP. IV, NATURAL SELECTION. 93 When our plant, by this process of the continued preservation or natural selection of more and more att~active flowers, had been rendered highly attractive to Insects, they would, unintentionally on their part, regularly carry pollen from flower to flower; and that they can most effectually do this, I could easily show by many striking instances. I will give only one-not as a very striking case, but as likewise illustrating one step in the separation of the sexes of plants, presently to be alluded to. Some holly-trees bear only male flowers, whi~h have four stamens producing rather a small quantity of pollen, and a rudimentary pistil; other holly-trees bear only female flowers; these have a full-sized pistil, and four stamens with shrivelled anthers in which not a grain of pollen can be detected. Ravin~ found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken fron1 different branches, under the microscope, and on all, without exception, there were pollen-grains, and on some a profusion of pollen. As the wind had set for several days from the female to the male tree, the pollen could not thus have been carried. The weather had been cold and boisterous, and therefore not favourable to bees, nevertheless every female flower which I examined had been effectually fertilised by the bees, accidentally dusted with pollen, having flown fi'OID tree to tree in search of nectar. But to return to our imaginary case: as soon as the plant had been rendered so highly attractive to insects that pollen was regularly carried from flower to flower, another process might commence. No naturalist doubts the .advantage of what has been called the "physiological division of labour;" hence we n1ay believe that it would be advantageous to a plant to produce stamens alone in one flower or on one whole plant, and pistils alone in |