566 Mr. C. F. M. Swynnerton on 



sometimes held upraised so as to show its under-surface and 

 the portion of the lower mandible that in other birds is 

 concealed by the tongue. This gives scope for markings 

 under the tongue, and that these in some cases exist is 

 shown by Mr. Bates's remarks on Spermestes poensis. It 

 is probably quite a nice instance of the exception that proves 

 the rule, for in birds' mouths generally, with the under- 

 surface of the tongue concealed, there is a lack there, not 

 only of spots, but of pigmentation generally, just as there is 

 on the extreme under-surface of the whole nestling of 

 Centropus burchelli, and on the little-shown under-surfaces 

 of many reptiles, small mammals, and arthropods, and this 

 through no consideration of counter-shading. The confine- 

 ment of mimicry to seen surfaces (e. g. in Dismorphia) is 

 exactly similar, and from the facts generally it can be fairly 

 argued that not only the spots, but the pigmentation 

 generally, of the mouths of birds must be for visual effect. 

 It can be argued from the same facts that whether we 

 regard the variations on which the coloration of mouths is 

 originally based as mutations or fluctuations, they can 

 hardly have been large. Strong pigmentation and spots 

 would be harmless under the tongue even if useless, and 

 were these to arise at all frequently by large and sudden 

 variation, we might expect to see them fairly frequently in 

 that position, just as the pigmentation of Dismorphia might 

 have been expected not to stop short exactly at the unseen 

 surface if the resemblance had arisen at a leap. Incident- 

 ally, the above and other facts connected with the coloration 

 of birds' mouths (e. g., the similarity between the breeding 

 and non-breeding mouths of male Pyromelana, in spite of 

 the extraordinary difi'erence in plumage) warn us, at any 

 rate, not to place too implicit a trust in explanations based 

 on correlation. It is also worth while adding that the 

 changes in the mouths are very slow and gradual. 



Some later Criticisms Answered. 

 I have lately received the following criticism : — "Among 

 insects a close inter-resemblance between individuals is 

 overwhelmingly the rule, whereas in birds' eggs, where 



