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 bring it down within easy reach of the net, especially if it be of the opposite sex." 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.

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.

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