5G4 Dr. F. A. Dixc^ rcj^/y to Mr. G. A. K. Marsliall 



But I venture to assert that this supposed case docs not 

 represent the usual, nor even a conniion condition of thinf^^s 

 in nature. This is no captious ohjection. I shall be able 

 to show that Mr Marshall's supposed case, though inter- 

 esting as an illustration of what might happen under 

 certain conceivable circumstances, is valueless as a supjwrt 

 of his position. 



In the first place, he postulates, on the part of his two 

 species of butterflies, A and B, the possession of " nauseous 

 ciualities in about the same degree." But every upholder 

 of Miillerian mimicry, so far as I am aware, is not only 

 ready to admit, but is prepared positively to assert that 

 distastefulucss is relative ; that it exists, like other means 

 of defence, in degrees that may vary indefinitely from 

 species to species. Any one who doubts this needs only to 

 refer to the experiments recorded by Mr. Finn in the 

 "Journal of the Asiatic Society of Bengal," 1895 and 

 1897, to say nothing of Mr. Marshall's own results as 

 published in the present and former papers (Trans. Ent. 

 Soc. Lond., 1902, pp. 297-390; also sujmi, pp. 128-130). 

 This cuts at the root of the statement that " a Miillerian 

 approach will only take place . . . from a rarer species 

 towards a more abundant one, and no species can in this 

 way approach another which has fewer individuals (and 

 therefore a higher percentage of loss) than itself" (p. 100). 

 On the contrary, there is every reason to think that 

 inferiority in numbers may be more than compensated by 

 a higher degree of distastefulness. 



The fact that different kinds of insect prey possess the 

 qualities of palatability or the reverse in different degrees, 

 and that these qualities are also relative to the likes and 

 dislikes of different enemies, is fully accepted and enlarged 

 upon by Mr. Marshall in a later section of his paper (pp. 

 128-130). But the strange thing is that he does not 

 recognise that this conclusion, so far from being alien to 

 F. Midler's theory, must form an integral part of any 



survival of each member of both A and B is ^5izJ/, but the chances of 



X 



survival after the defection of n are — 



For eacli member of A, "^ ; 



X — n 



For each member of B x + n — y, 



(including the variety of A), x + n 



the advantage of B over A of course increasing with increasing 



values of n. 



