VIII 
ORIGIN AND USES OF COLOUR IN ANIMALS 
227 
bring it down within easy reach of the net, especially if it be 
of the opposite sex .” 1 In a great number of insects, no doubt, 
form, motions, stridulating sounds, or peculiar odours, serve to 
distinguish allied species from each other, and this must be 
especially the case with nocturnal insects, or with those whose 
colours are nearly uniform and are determined by the need of 
protection ; but by far the larger number of day-flying and 
active insects exhibit varieties of colour and marking, forming 
the most obvious distinction between allied species, and which 
have, therefore, in all probability been acquired in the process 
of differentiation for the purpose of checking the intercrossing 
of closely allied forms . 2 
Whether this principle extends to any of the less highly 
organised animals is doubtful, though it may perhaps have 
affected the higher mollusca. But in marine animals it seems 
probable that the colours, however beautiful, varied, and 
brilliant they may often be, are in most cases protective, 
assimilating them to the various bright-coloured seaweeds, or 
to some other animals which it is advantageous for them to 
imitate . 3 
Summary of the Preceding Exposition. 
Before proceeding to discuss some of the more recondite 
phenomena of animal coloration, it will be well to consider 
for a moment the extent of the ground we have already 
covered. Protective coloration, in some of its varied forms, 
has not improbably modified the appearance of one-half of 
the animals living on the globe. The white of arctic animals, 
the yellowish tints of the desert forms, the dusky hues of 
crepuscular and nocturnal species, the transparent or bluish 
tints of oceanic creatures, represent a vast host in themselves; 
but we have an equally numerous body whose tints are 
adapted to tropical foliage, to the bark of trees, or to the soil 
1 Quoted by Darwin in Descent of Man, p. 317. 
2 In the American Naturalist of March 1888, Mr. J. E. Todd lias an 
article on “ Directive Coloration in Animals,” in which he recognises many of 
the cases here referred to, and suggests a few others, though I think he 
includes many-forms of coloration — as “paleness of belly and inner side of 
legs ” — which do not belong to this class. 
3 For numerous examples of this protective colouring of marine animals 
see Moseley’s Voyage of the Challenger, and Dr. E. S. Morse in Proc. of Bust. 
Soc. of Nat. Hist., vol, xiv. 1871. 
