(or other inferiority) on the part of mongrels or hybrids may clearly be seen to operate in this direction, is as a negative condition; not as an active cause. In the proportion that mongrels are impotent with one another, they will not so much compete with their parent forms for food, etc.; and in the proportion that they are impotent with their parent forms, they will not counteract any tendency which the latter may continue to develop in the direction of a still further segregation. If the mongrels are fully vigorous and fully fertile, both _inter se_ and with their parent forms, the effect will be to retard, if not altogether to prevent, any further progress of physiological separation between the parent forms; because the free intercrossing of the mongrels with one another, and also with their parent forms, will be continually supplying progeny in which the physiological peculiarity is either attenuated or altogether abolished. But this is quite a different thing from supposing that infertility (or inferiority) of the mongrels can react upon the generative system of the parent forms, so as to increase in them the physiological peculiarity on which their segregate breeding depends: infertility (or inferiority) of the mongrels is but a negative condition which favors the preservation of further degrees of this segregate breeding, if such further degrees should be induced by any other causes. Now, it does not appear that Mr. Wallace has clearly perceived this important distinction, because he throughout speaks of "this inferiority of the hybrid offspring as the essential point." Obviously, however, the essential point is the physiological variation in the parent forms, i. e., the original occurrence and subsequent development of infertility between the first crosses. Granting to Mr. Wallace, for the sake of argument, that this development could not proceed at all, were it not for the inferiority of the mongrels; still the inferiority of the mongrels need not be the cause of this development. Therefore it is most incorrect to say, "it must be particularly noted that this effect (i. e., increase of infertility between the parent forms) would result, not by the preservation of the infertile variations on account of their infertility, but by the inferiority of the hybrid offspring." "This effect" must be due to causes which act upon the generative systems of the parent forms, even though such causes might be _counteracted_ by the _withdrawal_ of the negative condition in question.