How animals are able to avoid poisonous plants

  Although the species of poisonous plants are comparatively few in number, the number of individuals is large. With so many poisonous plants occurring over a wide range, one might wonder how animal life has survived so well. The answer is that animals seem to be able, in some way that we do not understand, to avoid these dangerous forms of vegetation.
  It is an old belief that in the jungle, if a man eats only what he sees monkeys eat, he will run no risk from poisonous vegeta¬tion. Certainly, animals do seem to know instinctively which plants are safe for them to eat. As we have seen, they shun some plants because they give off an unpleasant odor. Even if poisonous plants do not give off any odor — or at least an odor that is perceptible to human beings — animals seem able to avoid them. Certain grazing animals also keep away from various mosses and ferns which, as far as we know, are not poisonous. We can only guess that such plants are harmful to the animals in some way.