64 INSECTS AND HUMAN WELFARE 



upon the forests and there is every probability that they will 

 be called upon in rapidly increasing measure. 



The problems entailed in maintaining forests in a produc- 

 tive state and those to be dealt with in rehabilitating de- 

 pleted ones, or in fostering the development of new ones, are 

 numerous and intricate, but manifestly they have much in 

 common with those encountered by the agriculturist. One 

 of the most striking differences is the element of time, due to 

 the slowness with which the lumber crop matures in com- 

 parison with the rapid development of the common annual 

 agricultural crops. This element of time at once exerts a 

 profound influence on the commercial aspect of the matter, 

 since the returns on the money invested in forests and that 

 spent in their maintenance are so delayed that they dwindle 

 in amount when compared with those which may be obtained 

 from investments of other kinds. Added to this, is a psy- 

 chological aspect due to the deep-seated human aversion for 

 rewards in the too distant future. These matters could be 

 dwelt upon at great length, but they are far from the present 

 discussion, except that t nev explain in great measure the 

 past and present relations between the human race and the 

 forests. One point which must be borne in mind, is that the 

 element of time bears also a very important biological re- 

 lation to the welfare of forests. This is least noticeable in 

 many primeval forests of mixed character, but becomes in- 

 creasingly significant in those of pure growth, and those 

 which are reproducing themselves after depletion through 

 extensive cutting, while it is of prime importance under the 

 conditions prevailing where extensive reforestation is under- 

 taken. 



We have pointed out, in connection with the insect enemies 

 of agricultural crops, the general relationship which exists 

 between these plants and insects. In the case of forest in- 

 sects, it will soon be seen how greatly the long develop- 

 mental period of trees affects the character of the depreda- 

 tions of insects that live at their expense. Another difference 

 which influences damage by insects, depends upon the devel- 



