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Show 104 NATURAL SELECTION. (CHAP. IV. ties, the supposed prototypes and parents of fu.ture wellmarked species, present slight and. ill-defined drfferen.ces. Mere chance, as we may call it, might cause one vanet~ to differ in some character fro1n its parents, and the off., spring of this variety again to differ from its parent in the very same character and in a greater degree; but this alone would never account for so habitual and large an amount of difference as that between varieties of the same species and species of the same genus. . As has always been my practice, let us seek light on this head from our domestic productions. We shall here find something analogous. A fancier is struck by a pigeon having a slightly shorter beak; another fJ.ncier is struck by a pigeon having a rather longer beak; and on the acknowledged principle that "fanciers do not and will not admire a medium standard,. but like extrmnes," they both go on (as has actually occurred with tun1bler pigeons) choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks. Again, we may suppose that at an early period one man preferred swifter horses ; another stronger and more bulky horses. The early differences would be very slight ; in the course of Hme, from the continual selection of swifter horses by some breeders, and of stronger ones by others, the differences would become greater, and would be noted as forming two sub-breeds; finally, after the lapse of centuries, the sub-breeds would become converted into two well-established and distinct breeds. As the differences slowly become greater, the inferior anhnals with intermediate characters, being neither very swift nor very strong, will have .been neglected, and will have tended to disappear. Here, then, we see in man's productions the action of what may be called the principle of divergence, causing differences, at first barely appreciable, steadily to increase, and the breeds to diverge in character both from each other and from their common parent. But how, it may be asked, can any analogous prn1Clple apply in nature~ I believe it can and . does apply most efficiently, from the simple circumstance that tJw more diversified the descendants from any one speCieS 0H.A.P. IV.] DIVERGENCE OF CHARACTER. 105 become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers. We can clearly see this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the nu1nber that can be supported in any country has long ago arrived at its full average. If its natural powers of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous. The 1nore diversified in habits and structure the descendants of our carnivorous animal beca1ne, the more places they would be enabled to occupy. What applies to one animal will apply throughout all time to all animals-that is, if they vary-for otherwise natural selection can do nothing. So it will be with plants. It has been experimentally proved, that if a plot of ground be sown with one species of grass, and a shnilar plot be sown with several distinct genera of grasses, a greater number of plants and a greater weight of dry herbage can thus be raised. The same has been found to hold good when first one variety and then several mixed varieties of wheat have been sown on equal spaces of ground. Hence, if any one species of grass were to go on varying, and those varieties were continually selected which differed from each other in at all the same manner as distinct species and genera of grasses differ from each other, a greater number of individual plants of this species of grass, including its modified descendants would succeed in living on the sa1ne piece of ground. And ~e well know t?at each species and each variety of grass IS annually sowing almost countless seeds; and thus, as it may be said, is striving its utmost to increase its numbers. Consequently, I cannot doubt that in the course of many thousands of generations, the most distinct varieties of any 5* |