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Show 84 DARWINISM C'IIAP. countenance, almo t as varied when closely studied n.s that of a human boino· · not only ca.n a shepherd distinguish every sheep in his fl;ck, but we <Lll know that each kitten in t~JC successive families of our old favourite cat has a fa.cc of Jt,' own with an expression and individuality distinct from :tll its brothers and sisters. Now this individual varia.bi I i ty exists among all creatures whatever, which we can. c1o:ely observe, even when the two parents arc very much ~hkc :tnd have been matched in order to preserve some spccw1 hn·ed. The same thing occurs in the vegetable kingdom. All plants raised from seed differ more or less from each other. In every bed of flowers or of vegetables we shall find, if we look closely, that there arc countless smn11 differences, in the siy,c, in the mode of growth, in the shape or colour of the lca.vcs, in the form, colour, or markings of the flowers, or in the ir.c, form, colour, or flavour of the fruit. These diff'ercnccs arc usually small, but are yet easily seen, and in their extremes are very considerable; and they have this important qnali ty, that they have a tendency to be reproduced, :mel thus l)y careful breeding any particular variation or group of v:Lria tions can be increased to an enormous extent- apparently to any extent not incompatible with the life, growth, and reproduction of the plant or animal. The way this is- done is by artincial selection, :1.nd it is very important to understand this process and its re. ult:-;. Suppose we have a plant with a small edible seed, :uHl we want to increase the size of that seeu. \V c grow :_ts large a. quantity of it as possible, and when the crop is ripe " o carefully choose a few of the very largest seeds, or we mty . by means of a sieve sort out a quantity of the largm;t :c •ds. Next year we sow only these large seeds, taking care to gi ' n them suitable soil and manure, and the result is fonnd to he that the avemge size of the seeds is larger th:w in the first crop, and that the largest seeds are now somewhat larger :uHl more numerous. Again sowing these, we obtain :t fnrtlH~r slight increase of size, and in a very few year. we obtain <L greatly improved race, which will always produce larger seeds than the unimproved race, even if cultivated without any special care. In this way all our fine sorts of vcgetahlrs, fruits, and flowers have been obtained, all our choice breed. IV VARIATION UNDER DOME TI 'A'l'ION 85 of cnJtlo or of poultry, our wonderful rae -horses, and our endless v:trictics of tlogs. It i. a very common but mi ·taken idea that this improvement is due to cros:in<r and feedin()' in h f . b b t c case o ammal·, and to improved cultivation in the case of plants. Cros. ing is occasionn.lly used in order to obtain a combination of qn:Liitics fonnd in two distinct breeds and also because it is fomul t~ increase the _ con titution:Ll vi~our; bnt every breed p~s essmg any exceptional quality is the result of the scle~t1on of variations occurring year after year and acc1:mulated m the J"?ttnner just described. Purity of breed, w1th repeated selectiOn of tho best varieties of that breed, is the foundation of all improvement in our domestic animals and cultivated phnts. P?·oofs of the Generality of Variation. Another very common error is, that varia,tion i. the exception, and rather a rare exception, and that it occurs only in one direction at a time-that is, that only one or two of the numerous possible modes of variation occur at the same time. The experience of breeders and cultivator., however prov~s that variation is the .rule instead of the exception, aml that 1t occurs, more or le. s, m almost every direction. This is shown by the fact that different species of plants and animals have required different kinds of modification to adnpt them to our use, and we have never failed to meet with variation in that particulcf!l' di1·ectio?1, so as to enable us to accnmulate it and so to produce ultimately a br(re amount of chan()'e in the required direction. Our gardens furnish us with n~mberlcss examples of this property of pbnts. In the cabbage and lettuce we have found vari:1.tion in the size and mode of growth of the leaf, enabling us to produce hy selection the almost innumerable varieties, some with solid heads of folia(l'c quite unlike a11y pbnt iu a state of natme others with curiou::;ly wrinkled lc:wcs like the savoy, oth~rs of a deep purple colour use<l for pickling. From the very same species as the cabbage (Bmssica oleracca) have ari::;en the broccoli and cauliflower, in which the leaves have undcrcrone little alteration, while the bmnching heads of flowers o·~ow into a compact m<Lss forming one of our mo. t delicate b vegetables. The brussels sprouts arc another form of the same plant, in |
