Studies on the Variability and Heritability of Pigmentation in Oenothera. 367 
is probably the correct one. Its appearance could not have been due 
to the loss of a “factor” from the germ plasm, because a certain 
number of reversions to O. rubrinervis regularly occur. Nor is it 
probable that its appearance was due to any “factor” having become 
latent and being reactivated in the O. rubrinervis offspring. Every- 
thing points to its being what it appears to be, namely, a positive 
variation in some physiological factor concerned in anthocyan for- 
mation. It is not necessary to think of the form as having lost an 
inhibitor, but rather has there been a quantitative readjustment of 
the relation between the substances which by their chemical inter- 
actions produce anthocyan, and those which decompose it as soon as 
formed, or which, by their presence divert the metabolic processes 
and bring about chemical reactions of a different sort. Miss WHELDALE 
(1910) believes, on the basis of her chemical studies, that anthocyan 
formation is dependent upon both glucoside-splitting and oxidising 
enzymes, while reductases may bring about the decomposition of 
anthocyan and thus act as inhibitors to prevent its appearance. The 
important point in the present case is that, whatever the processes 
concerned in anthocyan production, the appearance and behavior of 
O. rubricalyx cannot be explained on the basis of the presence or 
absence of any “factor” or substance in this mutant, which is not 
also present in O. rubrinervis. Obviously, the germinal change has been 
rather in a quantitative readjustment, and it would seem that 
many, though not necessarily all, the cases of Mendelian color in- 
heritance, are explainable on a similar quantitative basis rather than by 
a presence-absence hypothesis. We know that many reactions, especially 
in organic chemistry, may go in quite different directions according to 
the relative amounts of the reacting substances present. In the same 
way, the presence in the germ plasm, of varying quantities of certain 
substances (or the materials which under given conditions, produce 
those substances) may determine the complete or almost complete 
suppression of one type of reaction or metabolism, the process going 
instead in another direction, the end-product of which is a different 
substance. There is much evidence to show that the relative pro- 
portions of sugars, tannins, glucosides and various enzymes and 
reducing substances present, determine whether anthocyan shall be 
formed, and if so in what quantity. The conditions of the germ 
plasm which bring about a certain adjustment in the relative pro- 
portions in which these substances shall appear in ontogeny, are the 
things which are really inherited. In a form in which there is an 
