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Show 88 NAT URAL SELECTION. [CDAP. IV. which produced more and more pollen, and had larger and larger anthers, would be selected. When our plant, by this process of the continued P.reservation or natural selection of more and n1ore attractive flowers, had been rendered highly attractive to insects, they would, unintentionlly on their part, regularly carry pollen from flower to flower ; and that they can most effectually do this, I could easily show by Inany 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, which have four stamens producing rather a · small quantity of pollen, and a rudin1entary 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. Having found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers, taken from different branches, under the microscope, and on all, without exception, there were pollengrains, 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 fayourable to bees, nevertheless every female flower which I examined had been effectually fertilised by the bees, accidentally dusted with pollen, having flown from 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 l~bour;" hence we may 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 another flower or on another plant. In plants under culture and placed under new conditions of life, sometimes the male organs and sometimes the female organs become more or less impotent ; now if we suppose this to occur in ever so slight a degree under nature, then as pollen is CH.AP. IV.) NATURAL SELECTION. 89 already carried regularly from flower to flower and as a more complete separation of the sexes of our plant would ~e ~d.vantage?us on. the principle of the division of labour, Individuals wit~ this tendency more and more increased, would be continually favoured or selected until at last a complete separation of the sexes would be' effected. . L~t us now turn to the nectar-feeding insects in our Imaginary case: :ve ma:y suppose the plant of which we ~ave been slowly Increasing the nectar by continued selection, to be a common plant ; and that certain insects depended in main P.art on its nec.tar for food. I could give ma~:y facts, sho'Y:ng ~ow anxi?us bees are to save time; for Instance, then habit of cutting holes and sucking the n~ctar at the .bases of certain flowers, which they can, With. a very httle ;more trouble, enter by the 1nouth. Bearrng suc_h facts In mind, I can see no reason to doubt that an a?cidental deviation in the size and form of the body, or_ In t~e curvature an~ length of the proboscis, &c., far too siight to be appreciated by us, might profit a bee or other Insects, so that an individual so characterised would be able to obtain its food more quickly and so have a better chance of Hving and leaving desc~ndants. I~s .desce~dants w?uld probably inherit a tendency to a Similar sh~ht deviation of structure. The tubes of the c.orollas of the common red and incarnate clovers (Trifohum pratens.e an~ incarnatum) do not on a hasty glance appear to differ In length; yet the hive-bee can easily suck the nectar out of the incarnate clover, but not out of the common red clover, which is visited by humblebe~ s alone; so that whole fields of the red clover offer in vam an ab~nda~t supply of precious nectar to the hivebee. Thus I~ Inight be ~ great advantage to the hive-bee to h.ave a shghtly longer or differently constructed probosCis. On. t.lie other hand, I have found by experiment that the ~ertlhty of c~over greatly depends on bees visiting and movin~ par~s of the corolla, so as to push the pollen on to the stigmatic su~ace. Hence, again, if humble-bees were to become rare In any country, it might be a great advanta~e. to the red cl.over -to have a shorter or more deeply divided tube to 1ts corolla, so that the hive-bee |