Man’’s chief offence against nature has been to damage the earth’’s natural covering of vegetation without replacing it with a system of farming able to maintain the fertility of the soil. Man the farmer penetrated new lands in many directions, and the forests were vulnerable to his various activities.
The roots of plants help to bind the all-important soil and keep it in place. Falling dead vegetation and animal remains ensure a regular return of nutrients to the soil. If kept in a good condition a layer of soil acts as a sponge and regulates the movement of water in the area. Also, green plants perform a further vital function on our planet. Carbon-dioxide is taken in by the leaves in the daytime because it is one of the raw materials needed for the making of food substances. At the same time oxygen is given out. The carbon-dioxide given out by the living world during respiration is thus used and turned into valuable plant products of all kinds. In this way a healthy balance of gases is maintained in our atmosphere.
As farming spread through Europe, Africa and Asia, this natural balance in the biological world was altered in a variety of ways. Cultivators needed to ensure that crops received maximum sunlight and rain and the minimum of competition from other plants. They therefore cleared the land as completely as possible of the previous vegetation.
The "slash-and-burn" method, as it is called, was developed very early in the history of agriculture, and present-day primitive Dyaks of Borneo, in the East Indies, still provide us with an example of this type of farming. They clear the tropical rain forest of their land with methods very similar to those used by the New Stone Age people in Europe five or six thousand years ago. The bark of the trees is cut so that they gradually die. Other vegetation is also cut down and, when it has dried out, is burned. This leaves gaps of bare soil between the dead trees, which now cast little shade. In these gaps the seeds are planted and from them harvests of a sort are finally removed.
After a year or two the harvest begins to get smaller and smaller because the plant food in the soil has been used up. The soil becomes exhausted and the community moves on the next area to deal with that in the same way. The forest soon invades the abandoned plots and fertility is restored. However, valuable soil will have been lost by erosion, and this is especially serious in hilly areas in the tropics where rainfall may be frequent and heavy. While populations were low and stable little damage was done, but with a steady increase in the size and number of human communities the forest and soil have little time to recover.
What’’s the author’’s attitude in the phrase "of a sort" ( Para. 4)