Variation

The method referred to is that of individual hill selection. In the former lesson, to which reference has already been made, it was shown that the value of selection depends on the amount of variation that can be found. In other words, selection as it should be viewed from

FIG. 124. Two tubers of Early Rose, showing variation in depth of eyes. (See Fig. 125.)

the practical standpoint is merely a process of isolation of strains already existing in the commercial variety. It is true that the majority of our commercial varieties of crops are composed of a mixture of several distinct strains, some of which are good while others are only mediocre or are poor. One eminent plant-breeder has termed these strains "elementary species." It can be readily seen that the yield per acre which any

DeVries, Hugo. The mutation theory. 1909.

crop will produce is merely the average performance record of all the different strains that may be present in the commercial variety.

The only way in which the amount of variation in a given variety of potatoes may be determined is by a test of individuals. We may choose two hundred tubers from a bin of potatoes of one variety. These tubers may appear very similar in regard to their uniformity, size, color, depth of eyes, and the like. Indeed, from external appearances we should judge

The characteristic of

that one of these tubers was just as good as another. This may be true so far as the desirability for present use as food is concerned. When we test the performance record of these two hundred parental tubers, however, we find that there is a great variation in regard to size, shape, color, eyes, and the like. Perhaps the most interesting variation will be noticed in the ability to yield. This test of indvidual variation may best be made by the use of the tuber-unit method, first suggested by Webber.5

Webber, H. J. Plant-breeding for farmers. Cornell University Agricultural Experiment Station. Bulletin 251. 1908. (This bulletin is out of print.)