216 TEOPICAL NATURE, AND OTHER ESSAYS. 



butterflies of the families Heliconidae and Danaidse, as 

 well as among the Nymphalidae and Erycinidse, many of 

 which obtain the necessary protection in other ways. 

 Among birds also, wherever the habits are such that no 

 special protection is needed for the females, and where 

 the species frequent the depths of tropical forests, and 

 are thus naturally protected from the swoop of birds of 

 prey, we find almost equally intense coloration ; as in 

 the trogons, barbets, and gapers. 



Local Causes of Colour-development. — Another real, 

 though as yet inexplicable cause of diversity of colour, 

 is to be found in the influence of locality. It is 

 observed that species of totally distinct groups are 

 coloured alike in one district, while in anather district 

 the allied species all undergo the same change of colour. 

 Cases of this kind have been adduced by Mr. Bates, by 

 Mr. Darwin, and by myself, and I have collected all the 

 more curious and important examples in my Address to 

 the Biological Section of the British Association, at 

 Glasgow in 1876 (Chap. VII. of this volume). The most 

 probable cause for these simultaneous variations would 

 seem to be the presence of peculiar elements or chemical 

 conjpounds in the soil, the water, or the atmosphere, 

 or of special organic substances in the vegetation ; and 

 a wide field is thus ofl*ered for chemical investigation 

 in connection with this interesting subject. Yet, how- 

 ever we may explain it the fact remains, of the same 

 vivid colours in definite patterns being produced in quite 

 unrelated groups, which only agree, so far as we yet 

 know, in inhabiting the same locality. 



Summary on Colour-development in Animals. — Let 

 us now sum up the conclusion at which we have arrived, 



