VIII 
ORIGIN AND USES OF COLOUR IN ANIMALS 
229 
a decrease of humidity, the paleness evidently resulting from 
exposure and the blanching effect of intense sunlight, and a 
dry, often intensely heated atmosphere. With the decrease of 
the aqueous precipitation the forest growth and the protection 
afforded by arborescent vegetation gradually also decreases, as 
of course does also the protection afforded by clouds, the 
excessively humid regions being also regions of extreme 
cloudiness, while the dry regions are comparatively cloudless 
districts.” 1 Almost identical changes occur in birds, and are 
imputed by Mr. Allen to similar causes. 
It will lie seen that Mr. Gould and Mr. Allen impute 
opposite effects to the same cause, brilliancy or intensity of 
colour being due to a brilliant atmosphere according to the 
former, while paleness of colour is imputed by the latter to 
a too brilliant sun. According to the principles which have 
been established by the consideration of arctic, desert, and 
forest animals respectively, we shall be led to conclude that 
there has been no direct action in this case, but that the effects 
observed are due to the greater or less need of protection. 
The pale colour that is prevalent in arid districts is in harmony 
with the general tints of the surface; while the brighter tints 
or more intense coloration, both southward and in humid 
districts, are sufficiently explained by the greater shelter due 
to a more luxuriant vegetation and a shorter winter. The 
advocates of the theory that intensity of light directly affects 
the colours of organisms, are led into perpetual inconsistencies. 
At one time the brilliant colours of tropical birds and insects 
are imputed to the intensity of a tropical sun, while the same 
intensity of sunlight is now said to have a “ bleaching ” effect. 
The comparatively dull and sober hues of our northern fauna 
were once supposed to be the result of our cloudy shies ; but 
now we are told that cloudy skies and a humid atmosphere 
intensify colour. 
In my Tropical Nature (pp. 257-264) I have called atten¬ 
tion to what is perhaps the most curious and decided relation 
of colour to locality which has yet been observed—the preval¬ 
ence of white markings in the butterflies and birds of islands. 
1 The “ Geographical Variation of North American Squirrels,” Proc. Best' 
Soc. of Xat. Hist., 1S74, p. 284 ; and Mammals and Winter Birds of Florida, pp 
233-241. 
