After a shower of rain look carefully in the garden or else on a lawn, common, or pasture field where the grass is closely grazed by cattle or does not naturally grow long, and you will find numbers of tiny heaps of soil scattered about. Carefully brush away a heap and a little hole is seen, now hit the ground near it a few times with a stick or stamp on it with your foot and the worm, if he is near the top, comes up. When he is safely out of the way dig carefully down with a knife or trowel so as to examine the hole or "burrow." At the top you generally find it lined with pieces of grass or leaves that the worm has pulled in; lower down the lining comes to an end, but the colour of the burrow is redder than that of the rest of the soil wherever the soil has a greenish tinge. These holes are useful because they let air and water down into the soil. [Illustration: Fig. 24. Soil in which earthworms have been living and making burrows] The following experiment shows what earthworms can do. Fill a pot with soil from which all the worms have been carefully picked out and another {55} with soil to which earthworms have been added, one worm to every pound of soil. Leave them out of doors where the rain can fall on to them. You can soon see the burrows and the heaps of soil or "casts" thrown up by the worms: these casts wash or blow over the surface of the soil, continually covering it with a thin layer of material brought up from below. Consequently the soil containing earthworms always has {56} a fresh clean look. After some time the other soil becomes very compact and is covered with a greenish slimy growth. When this happens carefully turn the pots upside down, knock them so as to detach the soil and lift them off. The soil where the earthworms had lived is full of burrows and looks almost like a sponge. Fig. 24 shows what happened in an experiment lasting from June to October. The other soil where there were no earthworms shows no such burrows and is rather more compact than when it was put in. Earthworms therefore do three things:-- (1) They make burrows in the ground and so let in air and water. (2) They drag leaves into the soil and thus help to make the mixture of soil and leaf mould. (3) They keep on bringing fresh soil up to the surface, and they disturb the surface so much that it is always clean and free from the slimy growth. All these things are very useful and so a gardener should never want to kill worms. The great naturalist, Darwin, spent a long time in studying earthworms at his home in Kent and wrote a very interesting book about them, called _Earthworms and Vegetable Mould_. He shows that each year worms bring up about 1/50th of an inch of soil, so that if you laid a penny on the soil now and no one took it, in 50 years it might be covered with an inch of soil. Pavements that were on the surface when the Romans occupied Britain are now covered with a thick layer of soil.