FORESTS AND THEIR USES. II 



selves. Observation has shown that the trunks of 

 trees in a wood, breast high, even at the hottest time 

 of the day, are five per cent, cooler than the air." 



This is sufficient for the direct evidence of the 

 influence of trees on climate, water, cultivation, famine, 

 and disease, but there are also collateral aspects which 

 present themselves to the naturalist, to which the 

 economist pays little attention. He sees in the 

 destruction of trees a great disturbance of the equili- 

 brium of animal life ; with the absence of woodlands 

 he takes cognizance of the absence of all forms of life 

 which depend upon trees for their existence. He 

 contemplates a depauperized flora and fauna. Whole 

 genera disappear, and sometimes the individuals of 

 certain species increase to an enormous extent, until 

 the artificial conditions, produced by an interference 

 with the harmonies of nature, become merged into a 

 new equilibrium in accordance with the new con- 

 ditions. 



No other class of animals is connected so closely 

 with the vegetable kingdom as insects ; very many of 

 them are not only destined to feed upon vegetables, 

 but even upon certain definite species or genera of 

 plants. " From this," writes Schouw, " it will readily 

 be concluded that forests are of great importance to 

 insects. Myriads of these little animals live upon and 

 in the trunks of trees, upon their leaves, flowers, fruits, 

 and upon their parasitical plants ; myriads of others 

 live upon the vegetable feeders. In temperate zones, 

 the number of insects is smaller than in the tropics^ 

 but nevertheless large enough, so that particular species 

 of insects sometimes destroy great tracts of forest ; 



