TKUE MIMICRY 111 



turnus a somewhat sharp separation of the two forms of female 

 has been evolved; the black, mimetic form, being the most fit, 

 has completely crowded the primitive yellow form out of the area 

 inhabited by Papilio philenor, while beyond this area, to the north 

 and west, the yellow form alone prevails. The extensive and careful 

 studies of Edwards have shown that the two forms occur together 

 only in a very narrow transition region. 



We thus see that the facts, wherever we scrutinize them carefully, 

 harmonize with the theory. Of course we can only penetrate to 

 a certain depth with the theory of selection, and we are still far 

 from having reached the fundamental causes of the phenomena. 

 Indeed, our understanding must in the meantime stop short before the 

 causes of variations and their accumulation, but up to that point the 

 theory gives us clearness, and discloses the causal connexion of 

 phenomena in the most beautiful way. Although we do not yet 

 understand how the southern female Pcqnlio turnus was able to 

 produce the advantageous black, we do see why a black variation, 

 when it did occur, should increase and be strengthened, until it 

 crowded out the yellow form from the area of the immune model, 

 and we are able in a general way to refer the whole complicated 

 phenomena of mimicry to their proximate causes. 



This is true also of other phenomena which have had no part in 

 establishing the theory, since attention was only directed to them 

 later, and it is true even of some which, at first sight, seem to contra- 

 dict the theory altogether. To this class belongs, for instance, the 

 phenomenon that immune species not unfrequently mimic each other, 

 as was first observed among the Heliconiid-like butterflies of South 

 America. In four diflferent families, the Danaidse, the Neotropidse, the ' 

 HeliconiidEe, and the Acraeidse, there are species, distributed over 

 the same area, which resemble each other in their conspicuous 

 colouring and marking, and also in the peculiar shape of the wings. 

 After what has been said one might be inclined to regard one of 

 these species as the unpalatable model and the others as the pala- 

 table mimics, but they are all unpalatable, and are not eaten by birds. 

 The puzzle of this apparent contradiction was solved by Fritz Miiller ^, 

 who pointed out that the aversion to non-edible butterflies is not 

 innate in birds, but must be acquired. Each young bird has to learn 

 from experience which victim is good to eat, and which bad. If 

 every inedible species had its particular and distinctive colour-dress 

 a considerable number of individuals of each species would fall victims 

 to the experiments of young birds in each generation, for a butterfly 



' Kosmos, vol. V, 1881, p. 260 onwards. 



