1.We sometimes hear of animals that have becomeextinct，such as the great auk，the last specimen ofwhich was shot during the present century.Plants，too，run the risk of becoming extinct in certain districts.When a rare plant is found in some nook among themountains，botanistsare so eager to get a specimenof it that sometimes not a single plant is left.There is no danger of our cultivated plants becoming extinct，however，for farmers take care to preserve a sufficient quantity of seed every year.
2.The lower animals are not so intelligent as to preserve or cultivate the plants on which they feed.
Where animals of the graminivorous or browsingtypeare plentiful，therefore，their feed-plants run the risk of being eaten up entirely，while they are young and juicy，and before the seeds of another crop have been sown.
3.Plants have therefore to rely on certain means of self-defence，in order that they may live their lives and sow their seeds，instead of being eaten up bodilyby the first grass-eating animal that happens to pass.Those that have no means of self-defense will naturally be eaten up first，while those with the best weapons or the toughest armour will live longest and produce most seed，and so will spread most widely over the ground.
4.The ways in which plants protect themselves from animals are often most ingenious and effective.Some plants contain a poisonous juice，and if an animal does not take warning from its taste or smell，his attack on the plant will cost him his life.A few poisonous plants grow in this country-the deadly nightshade，the water-hemlock，andothers；and children have sometimes been poisoned by eating such plants.There are others，such as water-cress，which we find very good to eat，but which aretoo bitter for animals to feed on.
5.It is a curious fact that the deadly nightshade，which is so poisonous to the browsinganimals that might otherwise eat it all up，is quite harmlessto beetles and smaller creaturesthat live on it.The plant can easily spare as much of its leaves as they require，and they seem rather to like its juice.Animals appear to detect poisonous plants by their smell，though there are some of them which to us have no smell at all.
6.Next to poisonous plants we might place sting-ing plants，such as the nettle.The leaves of the nettleare studdedwith fine hollow hairs，rounded at the tip.
When one of these hairs is touched，its round point breaks off，and its sharp，ragged edge pierces the skin，while the juice which filled the hollow flows into the wound.The sting of some of the huge tropical plants of this kind is nearly asdangerous as the bite of a poisonous snake.Yet the nettle can hardly be called poisonous，for the young s h o o t s a r e s o m e t i m e s used as food，and taste a little like spinach when they are boiled.