Appendix A. 153 



The paper likewise indicates a point which, in studying 

 Mr. Wallace's theory, I have missed. It will be remem- 

 bered that the only apparent difference between his theory 

 and mine has been shown to consist in this that while 

 I was satisfied to state, in a general way, that natural selec- 

 tion is probably able to increase a selective fertility which 

 has already been begun by other causes, Mr. Wallace 

 has sought to exhibit more in detail the precise conditions 

 under which it can do so. Now, Mr. Gulick shows that 

 the particular conditions which Mr. Wallace describes, even 

 if they do serve to promote an increase of cross-infertility, 

 are conditions which preclude the possibility of natural selec- 

 tion coming into play at all. So that if, under these parti- 

 cular conditions, a further increase of cross-infertility does 

 take place, it does not take place in virtue of natural selection. 

 To me it appears that this criticism is sound; and, if so, 

 it disposes of even the one very subordinate addition to 

 our theory which Mr. Wallace " claims " as the most 

 " distinctive " part of his. 



The following is the criticism in question : 



On pages 173-186 Mr. Wallace maintains that "Natural 

 selection is, in some probable cases at all events, able to 

 accumulate variations in infertility between incipient species" 

 (p. 174) ; but his reasoning does not seem to me conclusive. 

 Even if we grant that the increase of this character [cross- 

 infertility] occurs by the steps which he describes, */ is not 

 a process of accumulation by natural selection. In order to be a 

 means of cumulative modification of varieties, races, or species, 

 selection, whether artificial or adaptational [i.e. natural], must 

 preserve certain forms of an intergenerating stock, to the 

 exclusion of other forms of the same stock. Progressive 

 change in the size of the occupants of a poultry-yard may be 

 secured by raising only bantams the first, only common fowls 

 the second, and only Shanghai fowls the third year ; but this 

 is not the form of selection that has produced the different 

 races of fowls. So in nature, rats may drive out and supplant 



