566 Dr. F. A. Dixcy's rc2>hi to Mr. G. A. K. Marshall 



Mr. Marshall has dealt with this point at all, he has 

 relegated it, like the former one, to the closing passages of 

 his paper : and here again his argument suffers by reason 

 of the exclusion from consideration, in its appropriate 

 place, of what is really an important matter to be kept in 

 view by all who would gain a clear and comprehensive 

 grasp of the Miillerian hypothesis. 



These then are the main reasons why Mr. Marshall's 

 dictum about relative numbers cannot be accepted. Nor can 

 we very well amend his arithmetical presentation of the 

 case by restatement unless we assign numerical values, 

 which can only be hypothetical, to the factors which he 

 lias omitted. 



I think it will be seen that that part of the contention 

 which depends merely on relative numbers must be 

 withdrawn, and that my opponent must take his stand, if 

 at all, upon the relative percentage of loss. A difference 

 between species in this respect, by Mr. Marshall's own 

 showing, will tend to the productionof Miillerian mimicry ; 

 so that the only point on which I need join issue with him 

 is his statement that " equality [in this case meaning an 

 equal percentage of loss] effectively prevents the Miillerian 

 selection from producing any mimetic results " (p. 100). 

 The force of this contention is much weakened when we 

 remember that there is no reason why the percentage of 

 loss should remain constant while the individuals of a 

 given form increase or diminish in number. In fact, from 

 Mr. Marshall's own statement (p. 99) that " Midler's 

 hypothesis postulates that the absolute destruction is 

 practically constant for each group of different colours," 

 it follows that the percentage loss must necessarily vary 

 with every variation in the nvmibers of the group. Hence, 

 as has been shown above (p. 563, note), Mr. Marshall's 

 conclusion, even on his own data, is not quite correct. 

 But there is still another factor to be taken into account 

 which is sufficient to dispose of the objection altogether. 



The supposed examples of distasteful butterflies, A and 

 B, by hypothesis owe their survival to the possession of 

 warning characters which are ultimately learnt by enemies 

 and avoided when these latter have become sutiiciently 

 experienced. To employ Prof. Poulton's useful term, A 

 and B are each of them provided with an aposeme ; A's 

 aposeme, also by hypothesis, being different from B's, and 

 the two not being liable to be mistaken for one another. 



