Snakes cannot be credited with much intelligence or educability, nor do they display any very marked instincts. The least stupid and most easily tamed are the species of the genera _Coluber_ and _Coronella_. There is, however, considerable difference in this respect between individuals of the same species. Most snakes, when freshly caught, defend themselves by biting, and some individuals retain their savage temper after months of captivity; others hardly ever bite, even if molested. The Common Grass-snake, for instance, hisses loudly and takes up a very threatening attitude, or even pretends to snap with open mouth, but very seldom bites; its principal defensive action when caught consists in voiding a most repulsive secretion from its anal glands, which it evidently controls, as it ceases doing so when accustomed to being handled. The same snake also produces, during the spring, an oily exudation from the skin which has the same repulsive smell. Mr. H. N. Ridley has observed a Malay snake allied to _Tropidonotus_, _Macropisthodon rhodomelas_, to exude drops of a white viscid liquid from the skin of its neck, which is flattened out like that of a Cobra when in an attitude of defence, and he noticed that his dog, seizing the snake to worry it, foamed at the mouth as if he had been biting a toad. The hissing is produced by the rapid expulsion of air from the lungs through the trachea and the notch at the end of the mouth, which is kept shut at the time. Snakes provided with an epiglottis (see p. 79) produce a much louder hissing. Other sounds are produced by some snakes. Thus, the Indian and African Vipers of the genera _Echis_ and _Cerastes_ make a curious, prolonged, rustling noise, by rubbing the folds of the sides of the body against one another. This sound is produced by friction between the serrated keels of the lateral scales, which are disposed obliquely with their tips directed downwards and backwards; the noise can even be repeated after the death of the animal, by twisting the body and thus rubbing or rasping these little saws against one another. The same thing probably takes place in the African genus _Dasypeltis_, in which we find a similar arrangement of the scales, though to a less degree.