98 Mr. G. A. K. Marshall on Diaposeniatism, with reference 



same area the educatioa of young birds, lizards, etc., 

 necessitates the destruction of approximately 1000 indi- 

 viduals in each group of distinctive patterns. Again, let 

 us suppose that A is a common species and is represented 

 by 100,000 individuals in that locality, while B is more 

 scarce and has only 5000. Now, ex liypotltesi, the 

 Miillerian factor will exact an equal toll from the two 

 species, and its selective importance must therefore 

 depend upon their relative numbers. In the present 

 instance the plentiful A will lose only 1 per cent, of its 

 individuals from this cause, while the scarcer B will lose 

 no less than 20 per cent. It is therefore reasonable to 

 suppose that if certain individuals of B chanced to 

 develop a variation in the direction of A, that variation 

 would have, in relation to the Miillerian factor, a definite 

 advantage over its own typical form, because it would 

 tend to share, at least to a small extent, in the relative 

 advantage enjoyed by A ; and the continuance of the 

 elimination would gradually tend to enhance the variety 

 and to diminish, and finally exterminate, the type. This 

 would be a case of simple Miillerian approach from B to 

 A. But if the mimicry is to be reciprocal it must be 

 shown that A is capable of approaching B by a precisely 

 similar process. In order to simplify the argument let us 

 make the supposition that 10,000 specimens of A simul- 

 taneously present a sudden marked variation in the 

 direction of B, to such an extent that young birds would 

 be liable to rank them with B rather than with A ; what 

 will be the effect of the Miillerian factor on this remark- 

 able variety ? The point again to be considered is the 

 relative incidence of the destruction. There will now be 

 90,000 examples of the A pattern, and 15,000 of the 

 B pattern. If 1000 individuals of each are again destroyed 

 by experimental tasting, a simple calculation shows that 

 the percentage of loss due to this factor will be six times 

 greater in the variety of A than in the typical form of A. 

 In fact the result is exactly the opposite of what took 

 place in the case of the variety from B towards A. 

 Whereas there we found that the variety had an appreci- 

 able advantage over its own type form, here it is seen to 

 be at a distinct disadvantage. It is obviously impossible 

 for the Miillerian factor to build up a mimetic resemblance 

 on such a foundation ; if it have effect at all, that effect 

 must be to gradually eliminate the variety and to establish 



