But it is different with that which is above the surface. He may indeed alter the climate a little by extensive drainage works or by planting forests, or cutting them down. But, on the whole, the action of the sun and the wind and the rain are an annuity fixed by nature for each plot of land. It is chiefly from them that the ownership of agricultural land derives its peculiar significance, and the Theory of Rent its special character. But the question how far the fertility of any soil is due to the original properties given to it by nature, and how far to the changes in it made by man, cannot be fully discussed without taking account of the kind of produce raised from it.
Human agency can do much more to promote the growth of some crops than of others.
Lastly, the kinds of produce which owe most to man's labour and care are the choicer kinds of fruits, flowers and vegetables, and of animals, particularly those which are used for improving their own breeds. Thus various then are the parts which man plays in aiding nature to raise the different kinds of agricultural produce. In each case he works on till the extra return got by extra capital and labour has so far diminished that it will no longer remunerate him for applying them. We are thus brought to consider the law of diminishing return. Such changes will be of vital importance when we come to draw inferences from the law of diminishing return, and particularly when we discuss the pressure of increasing population on the means of subsistence.