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the last edition of The Origin of Species was prepared ; and it is clear that Mr. Darwin himself did not fully recognise the enormous amount of variability that actually exists. This is indicated by his frequent reference to the extreme slowness of the changes for which variation furnishes the materials, and also by his use of such expressions as the following: “A variety when once formed must again, perhaps after a long interval of time, vary or present individual differences of the same favourable nature as before” (Origin, p. 66). And again, after speaking of changed conditions “affording a better chance of the occurrence of favourable variations,” he adds : “ Unless such occur natural selection can do nothing” (Origin, p. 64). These expressions are hardly consistent with the fact of the constant and large amount of variation, of every part, in all directions, which evidently occurs in each generation of all the more abundant species, and which must afford an ample supply of favourable variations whenever required ; and they have been seized upon and exaggerated by some writers as proofs of the extreme difficulties in the way of the theory. It is to show that such difficulties do not exist, and in the full conviction that an adequate knowledge of the facts of variation affords the only sure foundation for the Darwinian theory of the origin of species, that this chapter has been written.
VARIATION OF DOMESTICATED ANIMALS AND
The facts of variation and artificial selection-Proofs of the generality of
variation—Variations of apples and melons—Variations of flowersVariations of domestic animals—Domestic pigeons-Acclimatisation -Circumstances favourable to selection by man-Conditions favourable to variation-Concluding remarks.
HAVING so fully discussed variation under nature it will be unnecessary to devote so much space to domesticated animals and cultivated plants, especially as Mr. Darwin has published two remarkable volumes on the subject where those who desire it may obtain ample information. A general sketch of the more important facts will, however, be given, for the purpose of showing how closely they correspond with those described in the preceding chapter, and also to point out the general principles which they illustrate. It will also be necessary to explain how these variations have been increased and accumulated by artificial selection, since we are thereby better enabled to understand the action of natural selection, to be discussed in the succeeding chapter.
The facts of Variation and Artificial Selection. Everyone knows that in each litter of kittens or of puppies no two are alike. Even in the case in which several are exactly alike in colours, other differences are always perceptible to those who observe them closely. They will differ in size, in the proportions of their bodies and limbs, in the length or texture of their hairy covering, and notably in their disposition. They each possess, too, an individual countenance, almost as varied when closely studied as that of a human being; not only can a shepherd distinguish every sheep in his flock, but we all know that each kitten in the successive families of our old favourite cat has a face of its own, with an expression and individuality distinct from all its brothers and sisters. Now this individual variability exists among all creatures whatever, which we can closely observe, even when the two parents are very much alike and have been matched in order to preserve some special breed. 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 are countless small differences, in the size, in the mode of growth, in the shape or colour of the leaves, in the form, colour, or markings of the flowers, or in the size, form, colour, or flavour of the fruit. These differences are usually small, but are yet easily seen, and in their extremes are very considerable ; and they have this important quality, that they have a tendency to be reproduced, and thus by careful breeding any particular variation or group of variations 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 artificial selection, and it is very important to understand this process and its results. Suppose we have a plant with a small edible seed, and we want to increase the size of that seed. We grow as large a quantity of it as possible, and when the crop is ripe we carefully choose a few of the very largest seeds, or we may by means of a sieve sort out a quantity of the largest seeds. Next year we sow only these large seeds, taking care to give them suitable soil and manure, and the result is found to be that the average size of the seeds is larger than in the first crop, and that the largest seeds are now somewhat larger and more numerous. Again sowing these, we obtain a further slight increase of size, and in a very few years we obtain a 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 vegetables, fruits, and flowers have been obtained, all our choice breeds of cattle or of poultry, our wonderful race-horses, and our endless varieties of dogs. It is a very common but mistaken idea that this improvement is due to crossing and feeding in the case of animals, and to improved cultivation in the case of plants. Crossing is occasionally used in order to obtain a combination of qualities found in two distinct breeds, and also because it is found to increase the constitutional vigour ; but every breed possessing any exceptional quality is the result of the selection of variations occurring year after year and accumulated in the manner just described. Purity of breed, with repeated selection of the best varieties of that breed, is the foundation of all improvement in our domestic animals and cultivated plants.
Proofs of the Generality of Variation. Another very common error is, that variation is 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 cultivators, however, proves that variation is the rule instead of the exception, and that it occurs, more or less, in almost every direction. This is shown by the fact that different species of plants and animals have required different kinds of modification to adapt them to our use, and we have never failed to meet with variation in that particular direction, so as to enable us to accumulate it and so to produce ultimately a large amount of change in the required direction. Our gardens furnish us with numberless examples of this property of plants. In the cabbage and lettuce we have found variation in the size and mode of growth of the leaf, enabling us to produce by selection the almost innumerable varieties, some with solid heads of foliage quite unlike any plant in a state of nature, others with curiously wrinkled leaves like the savoy, others of a deep purple colour used for pickling. From the very same species as the cabbage (Brassica oleracea) have arisen the broccoli and cauliflower, in which the leaves have undergone little alteration, while the branching heads of flowers grow into a compact mass forming one of our most delicate vegetables. The brussels sprouts are another form of the same plant, in
which the whole mode of growth has been altered, numerous little heads of leaves being produced on the stem. In other varieties the ribs of the leaves are thickened so as to become themselves a culinary vegetable; while, in the Kohlrabi, the stem grows into a turnip-like mass just above ground. Now all these extraordinarily distinct plants come from one original species which still grows wild on our coasts; and it must have varied in all these directions, otherwise variations could not have been accumulated to the extent we now see them. The flowers and seeds of all these plants have remained nearly stationary, because no attempt has been made to accumulate the slight variations that no doubt occur in them.
If now we turn to another set of plants, the turnips, radishes, carrots, and potatoes, we find that the roots or underground tubers have been wonderfully enlarged and improved, and also altered in shape and colour, while the stems, leaves, flowers, and fruits have remained almost unchanged. In the various kinds of peas and beans it is the pod or fruit and the seed that has been subjected to selection, and therefore greatly modified ; and it is here very important to notice that while all these plants have undergone cultivation in a great variety of soils and climates, with different manures and under different systems, yet the flowers have remained but little altered, those of the broad bean, the scarlet-runner, and the garden-pea, being nearly the same in all the varieties. This shows us how little change is produced by mere cultivation, or even by variety of soil and climate, if there is no selection to preserve and accumulate the small variations that are continually occurring. When, however, a great amount of modification has been effected in one country, change to another country produces a decided effect. Thus it has been found that some of the numerous varieties of maize produced and cultivated in the United States change considerably, not only in their size and colour, but even in the shape of the seed when grown for a few successive years in Germany. In all our cultivated fruit trees the fruits vary immensely in shape, size, colour, flavour, time of ripening, and other qualities, while the leaves and flowers usually differ so little that they are hardly distinguishable except to a very close observer.
1 Darwin, Animals and Plants under Domestication, vol. i. p. 322.