22 PHOTOSYNTHESIS 



realization of the power of fire was slow in coming. Not until the middle 

 of the eighteenth century did man comprehend the value of the great 

 patrimony which had been stored for him in the earth. And then by an 

 intricate interplay of social and economic circumstances man-power was 

 suddenly replaced by steam-power. The spinners and weavers of England 

 were the first to abandon man-power as a prime mover, and then in a 

 hurried succession the kindling of coal-fires caused revolution in one in- 

 dustry after another. Through the production of energy in the form 

 of steam and its use in the smelting of ores, coal brought about the greatest 

 revolution in human history. 



Similarly petroleum, known to man in small quantities for centuries, 

 m recent years has brought about a revolution in his methods of trans- 

 portation. The great success of internal combustion engines in automo- 

 biles, airplanes, tractors, etc., as well as the many uses of the Diesel engine, 

 has very greatly influenced our economic life. Not only as fuel in the 

 many uses of the internal combustion engine but as the main source of 

 lubricants has petroleum become one of the most important commodities 

 on which our present civiUzation depends. So that in our modern life 

 the physical strength of man, the unit one-man power, is an almost in- 

 significant factor. A single machine can accomplish the work of a whole 

 army of men, so that not only have many burdens been lifted from the 

 shoulders of man, but the world's output of work has been enormously 



increased. 



Animate energy has to a very large extent been replaced by inanimate 

 laborers, and the true strength of man is measured by the intelligence he 

 exercises in utilizing inanimate forces. For this reason scientists have 

 for some time been occupied with the problem of our resources of inani- 

 mate energy. Thus already in the last century the eminent physicist Bolz- 

 mann pointed out that the struggle for existence is essentially not a fight 

 for raw materials which are abundant in earth, sky and sea, nor for the 

 energies as such, but for the potential energies as in coal, oil, sugar and 



meat. 



While, then, to a considerable extent there is a strong tendency to 

 emancipate man from his role as a beast of burden, he still requires his 

 daily ration of energy. In fact, all living things on the earth demand for 

 their maintenance and propagation a continuous supply of energy. The 

 source of this energy for living things is derived from food. All animals, 

 including man, are fundamentally dependent upon plants for their food. 

 The object of agriculture is essentially to provide man with such materials 

 from which he is able to derive the energy necessary for the maintenance 

 of his bodily activities, his growth and propagation. 



So far as the matter is concerned of which the food material is com- 

 posed, there exists a closed cycle. Man partly feeds on animals, and ani- 

 mals on plants ; the plants feed on the carbon dioxide given to the 

 air by the animals as a result of the latter's use of food. Thus the plant 

 reconverts the waste products of animal metabolism into food. The latter 



