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132 flaming patch that seems to burst out of the centre of it—for the head is drawn right back upon the body—very largely contributes. Especially is this so when, as is mostly the case, there is considerable shadow in the recess of the nest, amidst the surrounding foliage. If it can be supposed that the large false head and painted eyes of the puss or elephant hawk-moth caterpillars have been acquired as a protection against enemies—as to which see Professor Poulton's interesting suggestion —then it certainly seems to me more than possible that the flame-like throat of the young cuckoo has been developed in the same manner, pari passu with the loud, snake-like hiss and intimidating gestures. In conclusion, I would suggest that the bright or pleasingly-coloured mouth-cavity which some birds possess may have a distinct meaning, and be the product either of natural or sexual selection.