Page:Scientia - Vol. X.djvu/125

Rh rage 30 eggs, for example). If the somatic condition - in this case the actual production record - were a good cri- terion of the behavior of the character in inheritance it would be expected that on the average the descendants of A would not be essentially different in egg producing capacity from the descendants of B. Precisely the same point is brought out in the work of Johannsen and Jennings. The selection of variations «within» the pure line produces no result so far as the progeny are concerned.

All of the experimental investigations referred to agree in showing in a most definite and indubitablc manner that there exist two distinct categories of variation, and that this fact must never be lost sight of in any discussion of heredity which is to lead to valid conclusions. On the one hand are the Variations which are definitely inherited (i. e., reappear in the progeny), presumably because they are in some way represented in the gcrininal substance; on the other hand are the purely somatic variations which do not reappear in the offspring and are not inherited, presumably because they are not represented in the germinal substance. Now the << law of ancestral inheritance » entirely disregards the existence of these two sorts of variations. In its fundamental thesis that the correlation between parent and offspring in regard to somatic conditions is a valid measure of the intensity of in- heritance it definitely and implicitly assumes that all varia- tions are of equal significance in heredity. Upon this funda- mental biological error, which is taken as a basic assumption, the whole superstructure of the biometric treatment of inhe- ritance is reared. When the significance and consequences of this initial error are perceived it is seen at once that the whole reasoning, so far as it concerns heredity, falls to the ground. Thus it is assumed that the existence of a dednite degree of correlation (say r = 0.40 ± 0.02) between parent and offspring indicates inheritance, providing both generations have been reared under reasonably the same environmental conditions, and an absence of correlation (r = 0) under similar circumstances means that the character studied is not inhe- rited. But the work of Johannsen and Jennings indicates that in general there is « no » correlation (r = 0) between parent and offspring « within the genotype » (i. e., within the same pure line). A10 we to conclude then that there is no inheritance