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Page:Popular Science Monthly Volume 65.djvu/213

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209
DE VRIES'S THEORY OF MUTATIONS.

tinguished: one, which is called fluctuating variability, oscillates round a mean value; we shall consider it a little more closely. Whichever characteristic of a species we happen to choose, we shall always find, in considering a number of specimens, that individual differences, individual variations, can be noticed which, when tabulated according to size or to number, do not exceed two opposite extremes. Half-way between these extremes we find the 'norm' for that particular characteristic. The fluctuation may be represented by a curve, the culminating point of which corresponds to the norms just mentioned, whereas to the right and to the left of it the curve gradually approaches the horizontal line and has a symmetrical shape. Quetelet and Galton have insisted on the great significance of the fact, that fluctuating variation remains enclosed within the limits of such a curve of regular shape; the curve itself is, therefore, often called Galton 's curve.

Not only for plants, but also for animals and especially for man, the existence of such Galton curves, expressing the amount of variability, has been definitely established by different observers in very numerous instances. Thus, for example, Ammon has obtained his material from South German recruits. We need not insist on the fact that the greater the number of cases, the more reliable the curve.

The different degrees of fluctuating variability can undoubtedly be seized upon by any one who wishes to make them the starting-point for the breeding of certain distinct variations. Thus, for instance, by constantly selecting for the reproductive process those plants in which a given deviation is strongly marked, after a certain time and after a series of generations, a plant can be obtained for which the Galton curve would indicate a displacement of its culminating point in the direction of the selected variation. In this way an increase in the yield of sugar obtained from the beet roots has been arrived at from about 7 per cent, to 13 or 14 per cent. Thus also ears of maize have been produced that bore 20 rows of grain, whereas the kind from which the experiment had started always bore 12 to 14 rows.

As soon, however, as such conscious and voluntary selection ceases, the next generations successively return to the original curve. In order to prevent this retrograde process, without a constant and repeated application of the artificial selective process, we are obliged to prevent the appearance of new generations, by forcing the plant to reproduce itself not by seed, but asexually by means of buds. It is well known that definite kinds of delicate fruit are reproduced in this fashion, because if multiplied by seed, they would always tend to fall back into their former state of less value. Transposing the culmination of the curve of variability artificially, as explained, or breeding variations to the right or to the left of the norm, can never exceed certain limits. Agencies are at work there which prevent the fluc-