Mr. Wallace's criticism of the theory of Physiological Selection is unsatisfactory; (l) because he has accepted the fundamental principle of that theory on pages 173-9, in that he maintains that without the cross-infertility the incipient species there considered would be swamped; (2) because he assumes that physiological selection pertains simply to the infertility of first crosses, and has nothing to do with the infertility of mongrels and hybrids; (3) because he assumes that infertility between first crosses is of rare occurrence between species of the same genus, ignoring the fact that in many species of plants the pollen of the species is pre-potent on the stigma of the same species when it has to compete with the pollen of other species of the same genus; (4) because he not only ignores Mr. Romanes' statement that cross-infertility often affects "a whole race or strain," but he gratuitously assumes that the theory of Physiological Selection excludes this "racial incompatibility" (which Mr. Romanes maintains is the more probable form), and bases his computation on the assumption that the cross-infertility is not associated with any other form of segregation; (5) because he claims to show that "all infertility not correlated with some _useful_ variation has a constant tendency to effect its own elimination," while his computation only shows that, if the cross-infertility is not associated with some form of _positive_ segregation, it will disappear[60]; and (6) because he does not observe that the positive segregation may be secured by the very form of the physiological incompatibility.... Without here entering into any computation, it is evident that, e.g. the prepotency of pollen of each kind with its own kind, if only very slight, will prevent cross-fertilization as effectually as a moderate degree of instinctive preference in the case of an animal. [60] "Positive segregation" is Mr. Gulick's term for forms of homogamy other than that which is due to selective fertility. Of these other, or "positive" forms, natural selection is one; but as it is far from being the _only_ one, the criticism points out that utility is not the _only_ conserving principle with which selective fertility may be associated.