The making of sounds is a capacity possessed by many animals, small and big. Often it seems to have no particular significance, but, as in the case of the "humming" of bees and flies and the "droning" of beetles, is the necessary accompaniment of the vibration of the wings. But many animals make sounds as a "call," either to other individuals of their species, irrespective of sex, or more definitely as signals and appeals to the other sex, just as the luminosity which happens to accompany certain necessary chemical activities in the bodies of the lower animals has become specialized and utilized in the glow-worm and other higher forms as a signal and appeal. The rubbing of rough surfaces against one another is developed into a "stridulating organ" which we find in crickets, locusts, scorpions, spiders, and even in marine crustacea, and it is often specialized as a sexual appeal. The mere production of sound by tapping against wood is used by the little beetle, the death-watch, as a call, and is responded to by his mate with similar tapping. Such "tapping" is developed into a remarkable rhythmic vibrating sound by the birds called woodpeckers, and has its significance in courtship. But it is chiefly by the inspiration and expiration of air over vibrating cords or membranes called "vocal organs" that animals produce distinctive and musical sounds. In most cases such animals have a more general and simple "cry," which is not necessarily a sexual appeal, but addressed to comrades generally, and also a more elaborate cry or song which is primarily used by the male as an attraction in courtship, but has in the case of many birds been inherited from original male singers by the females also. The "singing" of birds--apart from simpler cries and calls--is a sexual address, an act of courtship. It is a display of power and capacity on the part of the male, and that such is its character is shown by the competition between male birds in the endeavour to "out-sing" one another. Some birds become extraordinarily excited in these competitions, which take the place of actual fighting, the victor who silences his opponents being the winner of the female bird, who is at hand listening to the competition. Caged chaffinches are celebrated for their eagerness to compete with one another in singing. They deliver their little song alternately until one is exhausted and unable to take up his turn. He is vanquished. So excited do the birds become that it occasionally happens that one of the competitors drops down dead. The beginning and directive causes of the particular song of different kinds of birds is not understood. But it is well known that they have a great gift of imitation. Parrots, piping crows, ravens, and other such birds are familiar instances, whilst little birds such as bullfinches can be trained to whistle the melodies which human beings have invented. Even the house-sparrow, which, though allied to singing finches, never sings