XXVlll 



will lose 1200 individuals ; l)ut if they are deceptively alike, then this loss 

 will be divided among them in proportion to their numbers, the first (A) will 

 lose -200, and the second (B) 1000. The former (A) accordingly gains 

 1000 (or 50 per cent.) of the total loss, and the latter (B) only 200 (or 

 2 per cent.) of this number. Thus, whilst the relative number of the two 

 species is in the ratio 1 : 5 the advantage derived by those possessing 

 the resemblance is 25 : 1. 



"If two species are concerned, of which the one is very common and 

 the other very rare, then the advantage falls almost entirely on the rarer 

 species. If, for example, Acrma Thalia were a thousand times commoner 

 than Eueides Favana, the latter would derive a million times greater benefit 

 from the resemblance of the two species, whilst for the Acraa the benefit 

 s practically nil. Thus Eueides Pavaiia might by natural selection be 

 converted into one of the most exact mimics of Acraa Thalia, although it 

 is just as distasteful as the species imitated, 



•' On the other hand, if two or even several distasteful species are about 

 equally common, resemblance brings them a nearly equal advantage, and 

 each step which the other takes in this direction is preserved by natural 

 selection — they would always meet each other numerically so that finally 

 one would not be able to say which of them has served as the model for the 

 others. In this manner are explained those cases where several allied 

 distasteful species (e.g., Colcenis Julia, Eueides Aliphera, and Dione Juno) 

 resemble one another — cases where such resemblance cannot be regarded 

 as inherited, and yet where neither of the species appears to claim to have 

 served as a model for the others. 



" To this category Ituna and Thyridia may belong, although the first 

 has probably made the greater step in passing from the former dissimilarity 

 to the present resemblance of the two species." 



With reference to Dr. Fritz MuUer's remarks on the inexperience of 

 young birds, Mr. Jenner Weir stated that from the numerous experiments 

 which he had made on the subject of larva3 which are eaten or rejected, he 

 had always been profoundly impressed with the utter disregard paid by birds 

 to larvae which were inedible. He had never but once seen a distasteful 

 larva even examined by a bird. When, day by day, he had thrown into his 

 aviary various larvse, those which were edible were eaten immediately ; 

 those which were inedible were no more noticed than if a pebble had been 

 thrown before the birds. It was Mr. Weir's opinion that the experience of 

 birds in this respect had become hereditary in the species, and was not the 

 result of the experience of individual birds, but was rather to be regarded 

 as an act of " unconscious cerebration." 



Mv. Bates, whilst acknowledging the great value of the numerous facts 

 adduced from his own personal observation by Dr. Fritz Miiller, could not 

 agree with liim in bis proposal to separate, as a distinct family, Itiiiia and 



