Of course the instinct continues to operate where it is no longer of any advantage. In summer, when the thistles are green, even when growing near the burrows, and the giant thistle (Carduus mariana) springs up most luxuriantly right on the mound, the vizcachas will not touch them, either disliking the strong astringent sap, or repelled by the thorns with which they are armed. As soon as they dry, and the thorns become brittle, they are levelled; afterwards, when the animal begins to drag them about and cut them up, as his custom is, he accidentally discovers and feasts on the seed: for vizcachas are fond of exercising their teeth on hard substances, such as sticks and bones, just as cats are of "sharpening their claws" on trees. Another remarkable habit of the vizcacha, that of dragging to and heaping about the mouth of his burrow every stalk he cuts down, and every portable object that by dint of great strength he can carry, has been mentioned by Azara, Darwin, and others. On the level plains it is a useful habit; for as the vizcachas are continually deepening and widening their burrows, the earth thrown out soon covers up these materials, and so assists in raising the mound. On the Buenos-Ayrean pampas numbers of vizcacheras would annually be destroyed by water in the great sudden rainfalls were the mounds loss high. But this is only an advantage when the animals inhabit a perfectly level country subject to flooding rains; for where the surface is unequal they invariably prefer high to low ground to burrow on, and are thus secured from destruction by water; yet the instinct is as strong in such situations as on the level plains. The most that can be said of a habit apparently so obscure in its origin and uses is, that it appears to be part of the instinct of clearing the ground about the village. Every tall stalk the vizcacha cuts down, every portable object he finds, must be removed to make the surface clean and smooth; but while encumbered with it he does not proceed further from his burrows, but invariably re-tires towards them, and so deposits it upon the mound. So well known is this habit, that whatever article is lost by night--whip, pistol, or knife--the loser next morning visits the vizcacheras in the vicinity, quite sure of finding it there. People also visit the vizcacheras to pick up sticks for firewood.