APPENDIX B. 



AK ExAMiNAnoN 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 " '. On the other hand, as 



1 His sentence, "all fertility not correlated with some useful variation 

 has a constant tendency to effect its own elimination," still further 

 restricts the possible action of physiological selection to cases where at 

 least one of the other forms of homogamy with which it is associated is 

 natural selection. Or, in other words, it is represented that physiological 

 selection must always be associated with natural selection, even if it be 

 likewise associated with any other form of exclusive breeding. But as 

 this further limitation appears to me self-evidently unjustifiable (seeing 



