In further discussion of these facts de Vries makes a suggestion which seems to me improbable. Since the egg-cells of _muricata_, for instance, bear a certain group of features which are missing on the male side, and conversely the pollen bears features absent from the female side, he is inclined to regard the _bad pollen grains_ as the bearers of the missing elements of the male side and to infer that there must similarly be defective ovules representing the missing elements of the female side. No consideration is adduced in support of this view beyond the simple fact that the characters borne by male and female are dissimilar, whereas it would be more in accord with preconception if the same sets of combinations were represented in each--as in a normal Mendelian case. There is as yet no instance in which the absence of any particular class of gametes has been shown with any plausibility to be due to defective viability, though there are, of course, cases in which certain classes of zygotes do not survive owing to defective constitution (_e. g._, the albinos of _Antirrhinum_ studied by Baur, and the homozygous yellow mice). I am rather inclined to suppose that in these examples of hybrids breeding true we shall find a state of things comparable with that to which we formerly applied the terms "coupling" and "repulsion." In these cases certain of the possible combinations of factors occur in the gametic series with special frequency, being in excess, while the gametes representing other combinations are comparatively few. In a recent paper on these cases Professor Punnett and I have shown that these curious results vary according to the manner in which the factors are grouped in the parents. If _A_ and _B_ are two factors which exhibit these phenomena we find that the gametic series of the double heterozygote differs according as the combination is made by crossing _AB × ab_, or by crossing _AB × aB_. In a normal Mendelian case the F_{1} form, _AaBb_, produces gametes _AB_, _Ab_, _aB_, _ab_, in equal numbers; but in these peculiar cases those gametes which contain Gametic series Number of Number of --------------------- gametes zygotes AB Ab aB ab in series formed