VIII ORIGIX AXD USES OF COLOUR IX AXLMALS 229 



a decrease of humidity, the paleness e^'idently 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." ^ Almost identical changes occur in birds, and are 

 imputed by Mr. Allen to similar causes. 



It M'ill be 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 coloiu's 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 skies ; but 

 now we are told that cloudy skies and a humid atmosphere 

 intensify coloui\ 



In my Trojncal Xafure (pp. 257-264) I have called atten- 

 tion to what is perhaps the most ciu'ious 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. 



^ The " Geographical Variation of Xorth American Squirrels," Pioc. Bost' 

 Sor. ofXat. Hist., 1S74, p. 2M;a.ndi Mammals and Winter Birds of Florida, ^t^- 

 233-241. 



