54 The Selection Theory 



The strongest of all proofs of the theory, however, is afforded by 

 cases of true "mimicry," those adaptations discovered by Bates in 

 1861, consisting in the imitation of one species by another, which 

 becomes more and more like its model. The model is always a 

 species that enjoys some special protection from enemies, whether 

 because it is unpleasant to taste, or because it is in some way 

 dangerous. 



It is chiefly among insects and especially among butterflies that 

 we find the greatest number of such cases. Several of these have 

 been minutely studied, and every detail has been investigated, so 

 that it is difficult to understand how there can still be disbelief in 

 regard to them. If the many and exact observations which have been 

 carefully collected and critically discussed, for instance by Poulton 1 , 

 were thoroughly studied, the arguments which are still frequently 

 urged against mimicry would be found untenable; we can hardly 

 hope to find more convincing proof of the actuality of the processes 

 of selection than these cases put into our hands. The preliminary 

 postulates of the theory of mimicry have been disputed, for instance, 

 that diurnal butterflies are persecuted and eaten by birds, but ob- 

 servations specially directed towards this point in India, Africa, 

 America and Europe have placed it beyond all doubt. If it were 

 necessary I could myself furnish an account of my own observations 

 on this point. 



In the same way it has been established by experiment and 

 observation in the field that in all the great regions of distribution 

 there are butterflies which are rejected by birds and lizards, their 

 chief enemies, on account of their unpleasant smell or taste. These 

 butterflies are usually gaily and conspicuously coloured and thus as 

 Wallace first interpreted it are furnished with an easily recognisable 

 sign : a sign of unpalatableness or warning colours. If they were 

 not thus recognisable easily and from a distance, they would fre- 

 quently be pecked at by birds, and then rejected because of their 

 unpleasant taste; but as it is, the insect-eaters recognise them at 

 once as unpalatable booty and ignore them. Such immune* species, 

 wherever they occur, are imitated by other palatable species, which 

 thus acquire a certain degree of protection. 



It is true that this explanation of the bright, conspicuous colours 

 is only a hypothesis, but its foundations, unpalatableness, and the 

 liability of other butterflies to be eaten, are certain, and its con- 

 sequences the existence of mimetic palatable forms confirm it in 

 the most convincing manner. Of the many cases now known I select 



1 Essays on Evolution, 18891907, Oxford, 1908, passim, e.g. p. 269. 



2 The expression does not refer to all the enemies of this butterfly ; against ichneumon- 

 flies, for instance, their unpleasant smell usually gives no protection. 



