When animals came upon the earth they ate the plants, and so the plants had to partly protect themselves to keep from being entirely destroyed. Thus some plants changed part of their stems or leaves into sharp thorns, as we see to-day in the hawthorns and cactuses. Some, like the mullein, covered their leaves with a disagreeable wooly substance that stuck to animals’ mouths and made them avoid the plants. These wooly coverings served two purposes,—regulated evaporation and protected from the attacks of animals. Some, like the aconite, manufactured a poisonous, disagreeable juice, while others, like the nettle, clothed the stems with stinging hairs. There are many, many ways by which plants have changed their leaves and stems in order to protect themselves from being eaten, and all this came about very, very gradually. While these things were happening, other things were happening too. Wherever there is life there is change. Living things keep changing all the time. The little fern that drops from the leaf of its parent is, in a general way, like the parent, but it is not exactly like its parent; it is _itself_ and has some peculiarities of its own. You see, it changes a little from the parent form or, as we say, varies. Every living thing has this power to vary within limits. No doubt, the power of variation was much greater in early times, and animals and plants were able to change much more then than now. As time went on, things sort of settled down, as it were, and stopped changing so rapidly. But way back in the early ages the plants changed a good deal. And all they had to work with, you will remember, was just stem and leaves,—not another thing. But that was enough. They could change stem and leaves into thorns, as we know, and they could do something else. They could change leaves into pistils. When the leaves divided their work, some plants devoted certain of their leaves to the task of making new plants. Ferns show this up to this very day. Look at a clump of ferns in the woods any time in the middle of the summer or later, and you will see that some of the fern leaves have little dark spots on their backs. Sometimes these dots are on their margins, sometimes on the ribs, and sometimes scattered everywhere over the back of the leaf. These dots are little cups filled with a fine dust, which falls on the ground and finally gives rise to more ferns. It is sometimes called fern seed, but the bits of dust are not exactly seeds. In the end they answer the same purpose, however. Well, suppose one of these fern leaves with the dots growing on it should curl over backwards until its edges met, and suppose the little grains should become true seeds, then we would have a very good ovary with the ovules inside. Fern leaves do not act in this way; they are too old-fashioned. But some of the leaves in flowering plants do. They just roll up into a pistil, with young plants, in the form of seeds, growing inside.