1901.] on Mimetic Insects. 695 



and Tliyridia together with the Dismorphias and moths mimicking 

 these, the group being selected from Bates's original examples.) 



The first step towards the necessary extension of the theory of 

 mimicry was taken by Fritz Miiller in 1879. He showed how the 

 principle of selection could be applied also to these cases if it is 

 admitted, as the evidence warrants, that the so-called " protected " 

 groups are only relatively and not absolutely exempt from persecution. 

 In other words, it is considered that young birds and other insect- 

 eaters do not inherit a knowledge of distasteful forms, but have to 

 acquire their knowledge by individual experience at the expense of 

 a certain amount of sacrifice of life by such distasteful forms. Fritz 

 Miiller, who had been a keen observer in Brazil for many years, liad 

 frequently noticed that the requirements of the Batesian theory were 

 not always complied with. In some cases the mimics appeared to be 

 more numerous than their models ; in other cases both mimic and 

 model belonged to protected groups, and he finally suggested the 

 above explanation with special reference to Ituna and Thyridia, which, 

 although not so closely related genetically as Miiller supposed at the 

 time, are undoubted mimics belonging to protected genera. The 

 fundamental requirement of the Miillerian theory, that even protected 

 species have to suffer a certain amount of persecution, was supported 

 by the observation that butterflies belonging to such groups were 

 frequently caught with mangled wings, indicating an attack by birds. 

 From the psychological side, it has since been proved by the observa- 

 tions of Lloyd Morgan on newly hatched birds, that the requirements 

 of the theory are quite in harmony with the facts. 



The advantage conferred upon protected species by bearing a 

 superficial resemblance to each other is capable of being expressed 

 (as Miiller did express it) algebraically, and is sufficiently explained 

 for ordinary purposes by the general statement, that if a number of 

 individuals belonging to different species have to be sacrificed, the 

 larger the number of species which resemble each other the smaller 

 the number of sacrifices undergone by each species relatively to its 

 whole number of individuals. Thus, while the Batesian theory might 

 be considered an extension of the theory of ordinary protecive 

 resemblance to cases in which the imitated object is another living 

 organism, the Miillerian theory might be considered an extension 

 of the theory of warning colours from individual species to groups of 

 species (" common warning colours " of Poulton). 



On the principles indicated, it becomes intelligible why, among 

 protected groups, there should so frequently prevail a general uni- 

 formity of type in colour and pattern. It is in such cases no longer 

 an individual type that insect enemies have by experience to learn 

 to avoid, but a generalised type of colour and marking, common to 

 whole groups. In illustration of this point, the Lecturer showed a 

 group of the British Vanessids in which there is extreme dissimilarity 

 of colour and pattern on the upper surface of the wings, there being 

 in this case no particular advantage conferred by a superficial re- 



