218 THE MECHANISM OF LIFE 



when, by reason of overcrowding and the absence of the neces- 

 sary mineral food substances, this continued multiplication is no 

 longer possible, dead vegetable substance in the form of woody 

 tissues, mould, seeds, leaves, oil, etc., accumulate and form 

 deposits of material of high calorific value. Some of these, in 

 the form of coal, and perhaps oil, remain throughout long periods 

 of geological time in a permanent, utilisable form. 



The fraction of the solar energy that is absorbed by green 

 plants, and is afterwards dissipated as waste, irrecoverable heat, 

 is very small. For these organisms are, in general, immobile, 

 and so their energy is not transformed into mechanical work, 

 which would become dissipated by friction into waste, low- 

 temperature heat. But in the animal the latter kinds of energy 

 transformations occur to a far greater extent than they do 

 among the plants. The animal is characteristically a machine 

 for the conversion of potential chemical energy into movement 

 of body and limbs, and this movement inevitably leads to friction, 

 and thus transforms into heat which becomes dissipated. Here 

 we are not considering the processes of reproduction ; if we were, 

 we should see that the tendency is, even in the animal kingdom, 

 for the indefinite multiplication of every species, and for the 

 distribution of the individuals over as wide an area of the surface 

 of the earth as possible. All animals, even the most slowly 

 breeding ones, are enormously prolific, and there seems to be no 

 limit to the numbers to which species may theoretically attain. 

 There is, of course, a practical limit which depends upon the 

 quantity of vegetable food substance available, which depends 

 again upon the area of land and sea which can be occupied by green 

 plants, and upon the quantities of the ultimate foodstuffs (chiefly 

 mineral compounds of nitrogen) that are available for the plants. 



If there were (say, as the results of volcanic activity) a con- 

 tinued supply of these indispensable food substances (nitrates., 

 nitrites, ammonia compounds, carbonic acid gas, and some other 

 mineral salts), there would seem to be no theoretical reason \\ liy 

 the quantity of available energy upon the earth in the form of 

 the organic substance of plants and animals should not steadily 

 accumulate. As it is, some substances which are the results of 

 organic activity do tend to accumulate; these are peat, lignite, 

 coal, perhaps oil, carbonate of lime in the form of coral reefs ;m<l 

 other limestone formations, silica in the form of diatom, and 

 radiolarian oozes and deposits, etc. Animal substances (fats, 



