92 THE EVOLUTION THEORY 



a mystery. The French Lepidopterist, Boisduval, went a step further 

 when he pointed out as something remarkable that nature sometimes 

 makes several species of quite different families exactly alike, and 

 called attention to three African butterflies, of which we shall have to 

 speak later in detail. But even he was too much fettered by the old 

 views of the immutability of species to arrive at a correct interpre- 

 tation. Thus it was reserved for Bates to take the decisive step. 

 Observing that the Heliconiidae occurred frequently, and usually in 

 large swarms, he concluded that they must have few enemies, and as 

 he never saw the numerous insectivorous birds and insects hunting 

 them, he further concluded that they must have something disagree- 

 able which secured them from the attacks of these predaceous forms. 

 On the other hand, he found that the heliconid-like Whites were 

 always rare, and he took this as a sign that they were much perse- 

 cuted, and that they must, therefore, be palatable tit-bits for the 

 insectivores. If it were possible, then, that a species of Whites with 

 the usual white colour of the family should give rise to variations, 

 which would make them in any degree resemble the Heliconiidse, 

 which are secure from persecution, and if, in addition, those that 

 exhibited the profitable variation attached themselves to swarms of 

 the mimicked form, then these variants would be to a certain extent 

 secured from attack, and more and more so in proportion as the 

 resemblance to the protected model increased. The great likeness of 

 these Whites to the Heliconiida?, Bates further argued, would depend 

 on a process of selection, based on the fact that, in each generation, 

 those individuals would on the average survive for reproduction 

 which were a little more like the model than the rest, and thus the 

 resemblance, doubtless slight to begin with, would gradually reach its 

 present degree of perfection. 



Bates's hypotheses have been subsequently confirmed in the most 

 striking way. The Heliconiidae do possess a disagreeable taste and 

 odour, and are utterly rejected by birds, lizards, and other animals. 

 It has been directly observed that puff-birds, species of Trogon, and 

 other insectivorous birds, looking down from the tops of trees in 

 search of food, allowed to pass unheeded the swarms of gaily coloured 

 Heliconiida3 which were fluttering among the leaves, and experi- 

 ments with various insectivorous animals yielded the same result: 

 the Heliconiidce are immune. We can, therefore, not only under- 

 stand that it must be advantageous to resemble them, we can also 

 appreciate many of their peculiar characters, such as their gay 

 coloration, which must serve as a sign of their disagreeable taste, and 

 their slow, fluttering flight, as well as their habit of flocking together, 



