the theory of Physiological Selection but that of Intensive Segregation[61] through Isolation, though he still insists on calling the process natural selection; for on page 183 he says, "No form of infertility or sterility between the individuals of a species can be increased by natural selection unless correlated with some useful variation, while all infertility not so correlated has a constant tendency to effect its own elimination." Even this claim he seems to unwittingly abandon when on page 184 he says: "The moment it [a species] becomes separated either by geographical or selective isolation, or by diversity of station or of habits, then, while each portion must be kept fertile _inter se_, there is nothing to prevent infertility arising between the two separated portions." [61] By Intensive Segregation Mr. Gulick means what I have called Independent Variability. The criticism proceeds to show yet further inconsistencies and self-contradictions in Mr. Wallace's treatment of this subject; but it now seems needless to continue. Nor, indeed, should I have quoted this much but for the sake of so fully justifying my own criticism by showing the endorsement which it has received from a completely independent examination. APPENDIX B. AN EXAMINATION BY MR. FLETCHER MOULTON OF MR. WALLACE'S CALCULATION TOUCHING THE POSSIBILITY OF PHYSIOLOGICAL SELECTION EVER ACTING ALONE. We have seen that the only important point of difference between Mr. Wallace's more recent views and my own on the problem of inter-specific sterility, has reference to the question whether variations in the way of cross-infertility can _ever_ arise and act "alone, in an otherwise undifferentiated species," or whether they can _never_ so arise and act. It is Mr. Wallace's opinion that, even if they ever do arise alone, at all events they can never act in differentiating a specific type, seeing that the chances against their suitable mating must be so great: only if they be from the first associated with some other form of homogamy, which will have the effect of determining their suitable mating, does he think that they can act in the way supposed by our theory of "selective fertility"[62]. On the other hand, as previously and frequently stated, I have so strong a belief in the segregating power of physiological selection, or selective fertility, that I do not think it is necessary for this principle to be _always___ associated with some other form of homogamy. From the first, indeed, I have laid great stress (as, also, has Mr. Gulick) on the re-enforcing influence which association with any other form of homogamy must exercise upon the physiological form, and vice versa; but I have also said that, in my opinion, the physiological form may in many cases be able to act entirely alone, or without assistance derived from any other source. The question here is, as we