382 
TROPICAL NATURE 
v 
here, not only is sexual selection out of the question, but 
the need for recognition and identification by others of the 
same species seems equally unnecessary. We can then only 
impute this variety to the normal production of colour in 
organic forms when fully exposed to light and air and under- 
going great and rapid developmental modification. Among 
more perfect animals, where the need for recognition has been 
added, we find intensity and variety of colour at its highest 
pitch among the South American butterflies of the families 
Heliconidae and Danaidse, as well as among the Nymphalidse 
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 another 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 will here give the more curious and important 
examples which I have been able to collect . 1 
The Influence of Locality on Colour in Butterflies and Birds 
Our first example is from tropical Africa, where we find 
two unrelated groups of butterflies belonging to two very 
distinct families (Nymphalidse and Papilionidse) characterised by 
a prevailing blue-green colour not found in any other continent . 2 
Again, we have a group of African Pieridse which are white 
or pale yellow with a marginal row of bead-like black spots ; 
and in the same country one of the Lycsenidse (Leptena 
1 These were first given in my Address to the Biological Section of the 
British Association at Glasgow in 1876. 
2 Romaleosoma and Euryphene (Nymphalidse), Papilio zalmoxis and 
several species of the Nireus-group (Papilionidse). 
