If we contrast these results with those obtained by picking at random a large number of leaves from different beech trees, we have no longer types of individuals, but racial characters. Pearson has given the following table to illustrate these points:
Frequency of Different Types of Beech Leaves
════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════ No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22 ────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼──── Frequency │ 1│ 7│ 34│ 110│ 318│ 479│ 595│ 516│ 307│ 181│ 36│ 15│ 1 ════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════
Thus the mode for beech trees in general is sixteen; but, as shown in the other table, this mode does not correspond with either of the two individual modes here ascertained. The illustration shows that the racial mode may differ from the individual mode. There are also cases known in which the mode of a group of individuals living in one locality is different from that of another group living in another locality. This difference may be a constant one from year to year, although so slight, that unless actual measurements are made, the difference cannot be detected, because of the overlapping of the individuals from different localities. If evolution took place by slow changes of this sort, it might be possible to detect its action, even when very slow, by means of measurements made on a large number of individuals. At least this has been suggested by those who believe new species may result from changes of this sort.
There is some evidence showing that by selecting particular individuals of a series, and breeding from them, the mode may be changed in the direction of selection. Thus it has been stated by Davenport that the descendants of twelve- and thirteen-rayed daisies give a polygon with a skewness of +1.92; while the descendants of twenty-one-rayed plants give a polygon with a skewness of -.13.
Pearson has described very concisely the possibilities involved in the selective action of the environment. He states that if we examine the frequency distribution of a set of organisms that have just become mature, and later make a similar examination on the same number of individuals (but not the same individuals) during the period of reproduction, we shall probably find that a change has taken place which may have been due to selection of some sort. The same thing might be found in the next generation, and, if it did, this would indicate that “selection does not necessarily mean a permanent or a progressive change.” The selection in this imaginary case would be purely periodic and suffice only to maintain a given race under given conditions. “Each new adolescent generation is not the product of the entire preceding generation, but only of selected individuals. This is certainly the case for civilized man, in which case twenty-six per cent of the married population produce fifty per cent of the next generation.”
