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Show 82 ~ATURAL SELECTION. [CHAP IV. mind that there are many unknown laws of correlation of growth, which, when one part of the organisation is modified through variation, and the modifications are accumulated by natural selection for the good of the being, will cause other modifications, often of the most unexpected nature. As we see that those variations which under domestication appear at any particular period of life, tend to reappear in the offspring at the same period ;-for instance, in the seeds of the Inany varieties of our culinary and agricultural plants; in the caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the colour of the down of their chickens ; in the horns of our sheep and cattle when nearly adult ;-so in a state of nature, natural selection will be enabled to act on and modify organic beings at any age, by the accumulation of profitable variations at that age, and by their inheritance at a corresponding age. If it profit a plant to have its seeds more and more widely dissmninated by the wind, I can see no greater difficulty in this being effected through natural selection, than in the cotton-planter increasing and improving by selection the do,vn in the pods on his cotton- trees. Natural selection may n1odify and adapt the larva of an insect to. a score of contingencies, wholly different from those whwh concern the mature insect. These modifications will no doubt affect, through the laws of correlation, the structure of the adult; and probably in the case of those insects which live only for a few hours, and which never feed, a large part of their structure is merely the correlated result of successive changes in the structure of their larvre. So, conversely, modifications in the adult will probably often affect the structure of the larva ; but in all cases natural selection will ensure that modifications consequent on other modifications at a different period of life, shall not be in the least degree injurious : for if they became so, they would cause the extinction of the species. Natural selection will modify the structure of the :r:oung in relation to the parent, and of the parent in relation to the young. In social animals it will adapt the CHAP. IV.] SEXUAL SELECTION. 83 structure of each individual for the benefit of the community; if each in consequence profits by the selected change. What natural selection cannot do, is to modify the structure of one species, without giving it any advantage, for th~ good of another spec~es ; and though statements to this effect may be found In works of natural history, I cannot find one case which will bear investigation. A structure used only once in an animal's whole life if of high importance to it, might be modified to any ext~nt by natur~l s.election; for instance, t~e great jaws possessed by certain Insects, and used exclusively for opening the cocoon-or the hard tip to the beak of nestling birds used for breaking the egg. It has been asserted, that of the best short-beaked tumbler-pigeons more perish in the egg than are able to get out of it; so that fanciers assist in the act of hatching. Now, if nature had to make the beak of a full-grown pigeon very short for the bird's own advantage, the process of modification would be very slow, a~d ~here would be simultaneously the most rigorous selection of the young birds within the egg, which had the n1ost powerful and hardest beaks, for all with weak beaks would inevitably perish: or, more delicate and more easily broken shells might be selected, the thickness of the shell being known to vary like every other structure. Sexual Seleotio~.-~as!lluch as peculiarities often appear under domestication In one sex and become hereditarily attached to that sex, the same fact probably occurs under nature, and if so, natural selection will be able to modify one sex in its functional relations to the other sex or in relation to wholly different habits of life in the tw~ sexes, as is sometimes the case with insects. .And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; tb.e result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, le~s rigorous than natural. selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. But in many |