Psendacraea ewyius hohleyi. 95 



by the late Colonel Maiiders on this very .snbject (Pioc. 

 Ent. Soc, p. xciii), that there is sufficient diiierence in 

 tone between the songs of individuals of the same species 

 on separate islands to show that some birds at least do 

 not roam from one to another, otherwise such insular 

 variations would not be perpetuated. I refer especially 

 to flycatchers {PJatystim and Tchi(rea), but it nmst be 

 admitted that the bee-eaters Merops and MeliUopluujus, 

 in particular the former, cross over quite wide areas of 

 water. 



In the case of Kome, however, we find a gradual decline 

 in the number of models and increase in mimics, with a 

 great proportional increase in the number of transitional 

 forms. 



The destruction of butterflies, so far as selective action is 

 concerned, is held to be mainly the work of young birds, 

 which have to learn what to eat and what to avoid. 



A young bird in 1914 would have found so many models 

 that any member of a combination would have been more 

 likely to be the relatively distasteful Acraeine; thus the 

 youngster would learn to leave that combination alone, 

 while Pseudacraea not conforming to the combination 

 would have been destroyed. As this bird grew older, 

 however, it w^ould have found the models becoming scarcer 

 and scarcer, and one nnist suppose that in consequence 

 the bird forgot what these models tasted like, so that in 

 later years no form of Pseudacraea had much more chance 

 of surviving than any other ; thus transitional forms would 

 begin to be preserved. 



A young bird hatched in 191G would have come into a 

 world where Planema and Psewlaemea were in approxi- 

 mately equal numbers, and thus its value as a selecting 

 agent would have been very limited. For it was just as 

 likely to meet a distasteful model or a relatively edible 

 Psendacraea, which might have been either a close mimic 

 or a transitional form. But as soon as Psendacraea became 

 more abundant than Planema the absence of selection would 

 come into play with ever-increasing force and transitional 

 forms would become abundant, perhaps only limited by 

 the range of variation and their physiological fitness. 



As to the reasons for the decline in numbers of Planema 

 on Kome from 1914 to 1918 I can give no satisfactory 

 suggestion. When I first went to Kome it seemed to me 

 that abundance of food-plant might account for the 



