However, the sting itself, viewed philosophically, is not without decided interest of its own. It is one, and perhaps the most highly developed, among the devices by which plants guard themselves against the attacks of animals. Weeds and shrubs with juicy, tender leaves are very apt to be eaten down by rabbits, cows, donkeys and other herbivores. But if any individuals among such species happen to show any tendency to the development of any unpleasant habit, which prevents the herbivores from eating them, then those particular individuals will of course be spared when their neighbors are eaten, and will establish a new and specially protected variety in the course of successive generations. It does not matter what the peculiarity may be, provided only it in any way deters animals from eating the plant. In the arum, a violently acrid juice is secreted in the leaves, so as to burn the mouth of the aggressor. In the dandelion and wild lettuces, the juice is merely bitter. In houndstongue and catmint it has a nauseous taste. Then again, in the hawthorn and the blackthorn, some of the shorter branches have developed into stout, sharp spines, which tear the skin of would-be assailants. In the brambles, the hairs on the stem have thickened into pointed prickles, which answer the same purpose as the spines of their neighbors. In the thistles, the gorse and the holly, once more, it is the angles of the leaves themselves, which have grown into needle-like points so as to deter animals from browsing upon them. But the nettle probably carries the same tendency to the furthest possible limit. Not content with mere defense, it is to some extent actively aggressive. The hairs which clothe it have become filled with a poisonous, irritating juice, and when any herbivore thrusts his tender nose into the midst of a clump, the sharp points pierce his naked skin, the liquid gets into his veins in the very neighborhood of the most sensitive nerves, and the poor creature receives at once a lifelong warning against attacking nettles in future.