Appendix B. 167 



show the converse of the problem— viz. the effect of swamping 

 where cross-fertilization is possible. I shall be very glad to 

 examine any one of these cases if you want me to do so ; 

 but I should prefer to leave it until I hear from you again. 



If you contrast the results that I have given above with 

 those given on pages i8i to 183 of Wallace's book, you will 

 see the enormous difference. His calculations can only apply 

 to the animal kingdom in those cases in which there is only 

 a union between one individual of each sex ; and before you 

 can deal with the question of such animals, you will have to 

 take into consideration many elements besides that of mere 

 fertility, if you wish to get any tolerably accurate result '. 



The above analysis leaves nothing to be added by me. 

 But, in conclusion, I may once more repeat that the particular 

 point with which it is concerned is a point of very subor- 

 dinate importance. For even if Mr. Wallace's computation 

 of chances had been found by Mr. Moulton to have been an 

 adequate computation — and, therefore, even if it had been 

 thus proved that physiological homogamy must always be 

 associated with some other form of homogamy in order to 

 produce specific divergence — still the importance of selective 

 fertility as a factor of organic evolution would not have 

 been at all diminished. For such a result would merely 

 have shown that, not only " in many cases " (as I originally 

 said), but actually in all cases, the selective fertility which 

 I hold to have been so generally concerned in the differentia- 

 tion of species has required for this purpose the co-operation 

 of some among the numerous other forms of homogamy. 

 But inasmuch as, by hypothesis, no one of these other or 

 co-operating factors would of itself have been capable of 

 effecting specific divergence in any of the cases where its 

 association with selective fertility is concerned, the mathe- 



' Here follows the Appendix presenting the calculations on which the 

 above results are founded ; but it seems unnecessary to reproduce it 

 on the present occasion. — G. J. R. 



