Page:The Zoologist, 4th series, vol 4 (1900).djvu/588

554 under favourable circumstances, is probably to reduce the number of enemies, this success being compensated, however, by the more persistent attacks of certain special enemies—the result being the same as in the cryptic colouring, namely, to keep up the average number of individuals.

13 Darwin remarks on the sound made by this species ('Voyage of the Beagle'), which he witnessed during his travels in South America. He believed that the sound was of sexual significance, and in his essay on sexual selection compared it to that made by the males of Halias prasinana during courtship—a sound which I have myself once heard. The display or exercise of secondary sexual characters is probably often a danger to the individual, although I fail to see how it is possible to argue from this that the cryptic colouring and attitudes of other phases of life are thereby rendered inoperative and valueless. The sound-producing time is one of high activity and rapid movement in both the species of Lepidoptera mentioned; in the case of the common English moth it is indulged in so rarely, that comparatively few naturalists have ever heard it, while in Ageronia it is not likely to be produced during more than a very small proportion of the life of the male. As to its cryptic colouring and, of even more importance, the corresponding instinctive attitudes and movements, Darwin made special remark in the volume already mentioned.

14 1 have noticed the same thing in North America. Not only was the distance very difficult to estimate, but the direction from which the sound came equally hard to trace.