120 CALIFORNIA ACADEMY OF SCIENCES. 



Allen's paper as quoted by Wallace would not be suffi- 

 cient to originate any new structure. He says: " It is 

 easy to see how natural selection could alter the general 

 size of the body, the relative sizes of parts, degrees of 

 colouration, etc., without encountering any great diffi- 

 culty from intercrossing. But if it were required to 

 produce, say, a fighting spur on a duck, clearly it could 

 not be done by natural selection alone, or when depend- 

 ing only on ' accidental variations,' " Mr. Romanes has 

 here granted far too much to his critic; for, if physio- 

 logical selection is not a necessary factor in the modifi- 

 cation of size, shape (relative sizes of parts), and color, 

 all closely related species wdiich depend upon such 

 features for their specific identity (and they constitute a 

 very large proportion of species), would be excluded from 

 the operation of the segregation of the fit. Nor would 

 this factor particularly assist in the production of spurs 

 and horns, for example. It is thought by many that 

 such structures are due to the factor of use, in which 

 case physiological selection would indeed become re- 

 stricted in its function. 



In replying to Wallace's criticisms of his view of 

 sterility between species, Mr. Romanes says: "Under 

 this head Mr. Wallace's criticism amounts to nothing 

 more than a vague suggestion to the effect that all other 

 naturalists may have hitherto exaggerated the generality 

 of some degree of sterility between species. But he 

 allows that it is 'a widespread phenomenon,' and gives 

 no reasons for differing from Mr. Darwin's careful esti- 

 mate of its frequency, he does not really furnish me 

 with any material to discuss." 



Mr. Seebohm in the introduction to his work entitled 

 The Geographical Distribution of Charadriidte, also 

 makes a sweeping criticism of physiological selection, 

 and as he and Mr. Wallace argue on the same lines upon 



