XXVI 1 



the Entomological Society llic remains of an old Indian collection which 

 had been destroyed by mites, &c., ' the surviving specimens all belonged 

 to protected genera {Exiplcen, Danais and Papilio), proving that the quality 

 which rendered these insects distasteful was, to a certain extent, retained 

 after death.'- 



"Now what does the mimicry of protected species signify? What 

 advantage can it be to the rare Eueides Pavana to be so wonderfully like 

 the common Acraa Thalia, and what benefit can one species derive from 

 resembling another, if each is protected by distastefulness. Obviously, 

 none at all if insectivorous birds, lizards, &c., have acquired bj' inheritance 

 a knowledge of the species which are tasteful or distasteful to them — if an 

 unconscious intelligence tells them what they can safely devour and what 

 they must avoid. But if each single bird has to learn this distinction by 

 experience, a certain number of distasteful butterflies must also fall victims 

 to the inexperience of the young enemies. Now if two distasteful species are 

 sufficiently alike to be mistaken for one another, the experience acquired 

 at the expense of one of them will likewise benefit the other; both species 

 together will only have to contribute the same number of victims which 

 each of them would have to furnish if they were different. If both species 

 are equally common, then both will derive the same benefit from their 

 resemblance — each will save half the number of victims which it has to 

 furnish to the inexperience of its foes. But if one species is commoner 

 than the other, then the benefit is unequally divided, and the proportional 

 advantage for each of the two species which arises from their resemblance 

 is as the square of their relative numbers. f For instance, let us suppose 

 that in a given region during one summer 1200 butterflies of a distasteful 

 species have to be destroyed before it becomes recognised as such, and that 

 in this region there exist JiOOO individuals of one (A) and 10,000 of 

 another (B) distasteful species. If they are quite diff"erent each species 



* ' Nature,' vol. xvi., p. 155. ' Kosmos,' i., p. 442. [Proc. Ent. Soc. 1877, p. xii.] 

 t Let a, and a^ be the numbers of two distasteful species of butterflies in 

 some definite district during one summer, and let Ji be the number of individuals 

 of a distinct species which are destroyed in the course of a summer before its 

 distastefulness is generally known. If both species are totally dissimilar, then each 

 loses n individuals. If, however, they are undistinguishably similar, then the 



first loses ^' " . and the second ^ . The absolute gain by resemblance is 



a, + a,2 «.i + «'2 



a, n a, n , . . ., 



therefore fur the first species n ', = -. ; and m a similar manner 



«■! + o^a *• + *2 



for the second, "''' ^^ . This absolute gain, compared with the occurrence of 

 a^ + flj 



the species, gives for the first, 1, =• — j^^ ,, and for the second species, 



^ ' ^ ''»,(«, + a,) 



1.=: — - — L -, whence follows the proportion, 1, : 1, = al : «?. 



«2 («i + ^s) 



