Curves to illustrate the influence of selection.

It may be objected that it is often with small differences rather than with the larger and more striking ones that the breeder is mainly concerned. It does not matter much to him whether the colour of a pea flower is purple or pink or white. But it does matter whether the plant bears rather larger seeds than usual, or rather more of them. Even a small difference when multiplied by the

size of the crop will effect a considerable difference in the profit. It is the general experience of seedsmen and others that differences of this nature are often capable of being developed up to a certain point by a process of careful selection each generation. At first sight this appears to be something very like the gradual accumulation of minute variations through the continuous application of a selective process. Some recent experiments by Professor Johannsen of Copenhagen set the matter in a different light. One of his investigations deals with the inheritance of the weight of beans, but as an account of these experiments would involve us in the consideration of a large amount of detail we may take a simple imaginary case to illustrate the nature of the conclusions at which he arrived. If we weigh a number of seeds collected from a patch of plants such as Johannsen's beans we should find that they varied considerably in size. The majority would probably not diverge very greatly from the general average, and as we approached the high or low extreme we should find a constantly decreasing number of individuals with these weights. Let us suppose that the weight of our seed varied between 4 and 20 grains, that the greatest number of seeds were of the mean weight, viz. 12 grains, and that as we passed to either extreme at 4 and 20 the number became regularly less. The weight relation of such a collection of seeds can be expressed by the accompanying curve (Fig. 30). Now if we select for

sowing only that seed which weighs over 12 grains, we shall find that in the next generation the average weight of the seed is raised and the curve becomes somewhat shifted to the right as in the dotted line of Fig. 30. By continually selecting we can shift our curve a little more to the right, i.e. we can increase the average weight of the seeds until at last we come to a limit beyond which further selection has no effect. This phenomenon has been long known, and it was customary to regard these variations as of a continuous nature, i.e. as all chance fluctuations in a homogeneous mass, and the effect of selection was supposed to afford evidence that small continuous variations could be increased by this process. But Johannsen's results point to another interpretation. Instead of our material being homogeneous it is probably a mixture of several strains each with its own average weight about

which the varying conditions of the environment cause it to fluctuate. Each of these strains is termed a pure line. If we imagine that there are three such pure lines in our imaginary case, with average weights 10, 12, 14 grains respectively, and if the range of fluctuation of each of these pure lines is 12 grains, then our curve must be represented as made up of the three components

A fluctuating between 4 and 16 with a mean of 10
B " " 6 " 18 " " 12
C " " 8 " 20 " " 14

Curves to illustrate the conception of pure lines in a population.

as is shown in Fig. 31. A seed that weighs 12 grains may belong to any of these three strains. It may be an average seed of B, or a rather large seed of A, or a rather small seed of C. If it belongs to B its offspring will average 12 grains, if to A they will average 10 grains, and if to C they will average 14 grains. Seeds of similar weight may give a different result because they happen to be fluctuations of different pure lines. But within the pure line any seed, large or small, produces the average result for that line. Thus a seed of line C which weighs 20 grains will give practically the same result as one that weighs 10 grains.