( Ixi ) 



immigrant 0. nigripennis. Prolonged isolation, in the 

 Hawaiian islands, from all the other dominant bearers 

 of the yellow-banded pattern also helped us to understand 

 the ultimate loss of the original pattern in so many of the 

 species. 



The mention of this great dominant Aculeate pattern made 

 it appropriate to refer at this point to a question raised by 

 Dr. Perkins in his paper — " Why should Colour-groups be 

 formed at all 1 Why is not the fact that an insect belongs 

 to the Aculeates sufficient warning by itself?" It might be 

 replied that the Aculeates themselves are probably avoided 

 for different reasons and in different degrees, and that, for 

 securing the advantages of Miillerian association, colour 

 and pattern are probably the most easily recognised and 

 remembered of all the characters that can be seen at a little 

 distance when an insect is at rest. There was furthermore 

 much, but not nearly enough, experimental evidence that 

 insect-eating animals were greatly impressed by the 'patterns 

 mimetic of the Aculeates. The methods of mimetic resemb- 

 lance were varied — sometimes the likeness was in pattern and 

 not in movement, sometimes in movement and not in pattern, 

 but in the most perfect examples there was likeness in both. 



Returning to the history of the Colour-groups in the islands, 

 we probably found, in the effects of occasional and accidental 

 inter-island migration, an answer to Dr. Perkins's further 

 difficulty based on the number of the Colour-groups, especially 

 on Oahu. Whatever may happen in the vast complexity of a 

 tropical continental area., we should certainly have expected, as 

 Dr. Perkins maintains, the persistence or formation of single 

 Miillerian Coloiir-groups on each of these small islands, 

 although we ought to be prepared for possible exceptions in 

 groups of specially associated species, such as the six forming 

 Colour-group III (= C) on Oahu, all of which were cajitured 

 at one time and in one spot by Dr. Perkins. Such special 

 associations may have all the effect of geographical isolation in 

 encouraging the growth of special warning patterns. Leaving 

 such possible exceptions on one side, we shouhl expect a single 

 Colour-group on a single island, but we should not expect the 

 same group to be formed iudepcndently in different islands, 



