按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
e a marked effect。 I strongly suspect that some well…known laws with respect to the plumage of male and female birds； in comparison with the plumage of the young； can be explained on the view of plumage having been chiefly modified by sexual selection； acting when the birds have come to the breeding age or during the breeding season； the modifications thus produced being inherited at corresponding ages or seasons； either by the males alone； or by the males and females； but I have not space here to enter on this subject。
Thus it is； as I believe； that when the males and females of any animal have the same general habits of life； but differ in structure； colour； or ornament； such differences have been mainly caused by sexual selection； that is； individual males have had； in successive generations； some slight advantage over other males； in their weapons； means of defence； or charms； and have transmitted these advantages to their male offspring。 Yet； I would not wish to attribute all such sexual differences to this agency： for we see peculiarities arising and becoming attached to the male sex in our domestic animals （as the wattle in male carriers； horn…like protuberances in the cocks of certain fowls； &c。）； which we cannot believe to be either useful to the males in battle； or attractive to the females。 We see analogous cases under nature； for instance； the tuft of hair on the breast of the turkey…cock； which can hardly be either useful or ornamental to this bird； indeed； had the tuft appeared under domestication； it would have been called a monstrosity。
Illustrations of the action of Natural Selection
In order to make it clear how； as I believe； natural selection acts； I must beg permission to give one or two imaginary illustrations。 Let us take the case of a wolf； which preys on various animals； securing some by craft； some by strength； and some by fleetness； and let us suppose that the fleetest prey； a deer for instance； had from any change in the country increased in numbers； or that other prey had decreased in numbers； during that season of the year when the wolf is hardest pressed for food。 I can under such circumstances see no reason to doubt that the swiftest and slimmest wolves would have the best chance of surviving； and so be preserved or selected； provided always that they retained strength to master their prey at this or at some other period of the year； when they might be compelled to prey on other animals。 I can see no more reason to doubt this； than that man can improve the fleetness of his greyhounds by careful and methodical selection； or by that unconscious selection which results from each man trying to keep the best dogs without any thought of modifying the breed。
Even without any change in the proportional numbers of the animals on which our wolf preyed； a cub might be born with an innate tendency to pursue certain kinds of prey。 Nor can this be thought very improbable； for we often observe great differences in the natural tendencies of our domestic animals； one cat； for instance； taking to catch rats； another mice； one cat； according to Mr。 St。 John； bringing home winged game； another hares or rabbits； and another hunting on marshy ground and almost nightly catching woodcocks or snipes。 The tendency to catch rats rather than mice is known to be inherited。 Now； if any slight innate change of habit or of structure benefited an individual wolf； it would have the best chance of surviving and of leaving offspring。 Some of its young would probably inherit the same habits or structure； and by the repetition of this process； a new variety might be formed which would either supplant or coexist with the parent…form of wolf。 Or； again； the wolves inhabiting a mountainous district； and those frequenting the lowlands； would naturally be forced to hunt different prey； and from the continued preservation of the individuals best fitted for the two sites； two varieties might slowly be formed。 These varieties would cross and blend where they met； but to this subject of intercrossing we shall soon have to return。 I may add； that； according to Mr。 Pierce； there are two varieties of the wolf inhabiting the Catskill Mountains in the United States； one with a light greyhound…like form； which pursues deer； and the other more bulky； with shorter legs； which more frequently attacks the shepherd's flocks。
Let us now take a more complex case。 Certain plants excrete a sweet juice； apparently for the sake of eliminating something injurious from their sap： this is effected by glands at the base of the stipules in some Leguminosae； 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 very vigorous seedlings； which consequently would have the best chance of flourishing and surviving。 Some of these seedlings would probably inherit the nectar…excreting power。 Those in individual flowers which had the largest glands or nectaries； and which excreted most nectar； would be oftenest visited by insects； and would be 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 pollen 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 nectar； and as pollen is formed for the sole object of fertilisation； its destruction appears a simple loss to the plant； yet if a little pollen were carried； at first occasionally and then habitually； by the pollen…devouring insects from flower to flower； and a cross thus effected； although nine…tenths of the pollen were destroyed； it might 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。
When our plant； by this process of the continued preservation or natural selection of more and more attractive 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； which 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 po