(4) The offspring as a group average lower in grade than their parents—that is, their mean regresses on that of the selected parents, but because of the higher mode about which variation occurs in each generation certain of the offspring are of higher grade than their parents. Thus an elevation of the grade of the parents in the next generation is made possible.
(5) With the selection of more extreme parents, the absolute regression of the offspring has not increased, but on the contrary has slightly diminished—that is, the advance made by the parents is retained by their offspring.
In [Table 15] have been brought together for comparison the means of the several horizontal rows of Tables 1 to 13. By examining the vertical columns of Table 15 the mean grade of the offspring of parents of a particular grade in any generation may be compared at a glance with that of parents of the same grade in any other generation. By running the eye down the columns, it will be observed that the mean grade of the offspring tends to increase upon repeated selection. Thus parents of grade 3.75 appear first in generation 4, the grade of their offspring being 2.75; the offspring of such parents in subsequent generations grade in order, 3.07, 3.22, 3.35, 3.49, 3.50, 3.69, 3.75, and 3.83 (twelfth generation not complete). The difference between parents and offspring in this series grows less and less and finally disappears altogether. If the grade of 3.75 parents in this series is compared with the grade of all offspring in the corresponding generations we have the following:
Table A.
| Generation. | Mean of offspring of 3.75 parents. | Mean of offspring of all parents. |
|---|---|---|
| 4 | 2.75 | 2.73 |
| 5 | 3.07 | 2.90 |
| 6 | 3.22 | 3.11 |
| 7 | 3.35 | 3.20 |
| 8 | 3.49 | 3.48 |
| 9 | 3.50 | 3.54 |
| 10 | 3.69 | 3.73 |
| 11 | 3.75 | 3.77 |
| 12 | 3.83 (35 individuals) | 3.94 (590 individuals) |
In generation 4 the 3.75 parents represented the most advanced individuals of the series, a whole grade in advance of the general average of the race. Their offspring showed a correspondingly large regression. The general average of the race steadily advanced in later generations until in generation 11 it equaled that of the 3.75 parents; then the regression vanished. In the following generation, 12 (which is still incomplete, but in which the average of the offspring thus far is 3.94), the 3.75 group of parents, which are now below the average of the race, actually produce offspring of higher grade than themselves, viz, 3.83. It will thus be seen that the regression is uniformly toward the mean of the race and changes its direction when that mean changes its position with reference to a particular grade of parents. This conclusion is supported by other columns of Table 15, but is best illustrated by this particular case because here the selection has extended over a greater number of generations than elsewhere in the series.
If one examines the horizontal rows of Table 15, he finds in general that numbers increase toward the right. Exceptions are commonest toward the ends of the rows where fewest individuals are represented. This increase means that, within any generation, as the grade of the parents rises, that of their offspring rises also. Since in general the selected parents are above the general average of the race for the time being, regression is naturally downward in nearly all cases.
From what precedes we may conclude (1) that in this series of rats the somatic character (appearance) of an individual is in general a true indication of its germinal character, since the higher the grade of the parents the higher the grade of the offspring, and vice versa; but that (2) the somatic character of an individual is not a perfect index of its germinal character, since the offspring of aberrant individuals are less aberrant than themselves, i. e., the offspring regress toward the mean of the race; yet that (3) by selection of plus variations we can displace, in a plus direction, not only the mean of the race, but also the upper and lower limits of its variation, the total amount of variability (standard deviation) being thereby only slightly decreased.
MINUS SELECTION SERIES.
This series begins with selected parents ranging in grade from -1.25 to -1.87. Their average, if each pair is weighted in proportion to the number of its offspring, is -1.46. The offspring ([Table 16]), like the offspring of the original plus selections, regress toward grade 0. They range in grade from +0.25 to -2.00, their mean being -1.00. The total number of offspring recorded in this generation is only 55, this being too small to warrant the calculation of a correlation coefficient.