170 OUTLINES OF FORESTRY. 



The principle of the survival of the fittest plays an 

 important part in preserving the balance of nature. 



Nearly every animal forms the food best fitted 

 to sustain the life of some other animal. In the 

 event of a too rapid multiplication of any particu- 

 lar form of life, some scourge or disease appears 

 which sweeps off the surplus and thus restores 

 nature's balance. 



As far as careful measurements have been made, 

 it can be safely assumed that the total value of the 

 solar radiation is practically the same now as it 

 was many thousands of years ago. Consequently, 

 the total amount of energy which the earth thus 

 receives from the sun, and which goes to maintain 

 the present mechanism of nature, is constant. 



The distribution of this solar energy is, however, 

 by no means constant. The general interchange 

 that is effected between the excessive heat of the 

 equator and the excessive cold of the polar regions 

 may take place rapidly or slowly, and thus produce 

 differences in the earth's general climate that not 

 infrequently give rise to a belief in a change in 

 the total heating power of the sun, when no such 

 change exists. For example : 



A bare, uncovered surface heats with extreme 

 rapidity, and consequently the air over it becomes 



