It is, however, in cutting down and burning forests of large trees that man has done the most harm to himself and the other living occupants of many regions of the earth's surface. We can trace these evil results from more recent examples back into the remote past. The water supply of the town of Plymouth was assured by Drake, who brought water in a channel from Dartmoor. But the cutting down of the trees has now rendered the great wet sponge of the Dartmoor region, from which the water was drawn all the year, no longer a sponge. It no longer "holds" the water of the rainfall, but in consequence of the removal of the forest and the digging of ditches the water quickly runs off the moor, and subsequently the whole countryside suffers from drought. This sort of thing has occurred wherever man has been sufficiently civilised and enterprising to commit the folly of destroying forests. Forests have an immense effect on climate, causing humidity of both the air and the soil, and give rise to moderate and persistent instead of torrential streams. Spain has been irretrievably injured by the cutting down of her forests in the course of a few hundred years. The same thing is going on, to a disastrous extent, in parts of the United States. Whole provinces of the Thibetan borders of China have been converted into uninhabitable, sandy desert, where centuries ago were fertile and well-watered pastures supporting rich cities, in consequence of the reckless destruction of forest. In fact, whether it is due to man's improvident action or to natural climatic change, it appears that the formation of "desert" is due in the first place to the destruction of forest, the consequent formation of a barren, sandy area, and the subsequent spreading of what we may call the "disease" or "desert ulcer," by the blowing of the fatally exposed sand and the gradual extension, owing to the action of the sand itself, of the area of destroyed vegetation. Sand-deserts are not, as used to be supposed, sea-bottoms from which the water has retreated, but areas of destruction of vegetation--often (though not always) both in Central Asia and in North Africa (Egypt, etc.), started by the deliberate destruction of forest by man, who has either by artificial drainage starved the forest, or by the simple use of the axe and fire cleared it away.