28 THE STUDY OF ANIMAL LIFE CHAP. 



increase in unchecked numbers, and destroy the crops of 

 grain and fruit. In a fuel-famine men have sometimes 

 been forced to cut down the woods which clothe the sides 

 of a valley, an action repented of when the rain-storms 

 wash the hills to skeletons, when the valley is flooded 

 and the local climate altered, and when the birds robbed 

 of their shelter leave the district to be ravaged by cater- 

 pillar and fly. American entomologists have proved 

 that the ravages of destructive insects may be checked 

 by importing and fostering their natural enemies, and 

 on the other hand, the sparrows which have established 

 themselves in the States have in some districts driven 

 away the titmice and other insectivorous birds thus 

 favouring the survival of pests. 



5. More Complex Interactions. The flowering plants 

 and the higher insects have grown up throughout long 

 ages together, in alternate influence and mutual per- 

 fecting. They now exhibit a notable degree of mutual 

 dependence ; the insects are adapted for sipping the 

 nectar from the blossoms ; the flowers are fitted for 

 giving or receiving the fertilising golden dust or pollen 

 which their visitors carry from plant to plant. The 

 mouth organs of the insects have to be interpreted in 

 relation to the flowers which they visit ; while the latter 

 show structures which have been fancifully called the 

 " foot-prints " of the insects. 



The relations of flowers and insects have formed the 

 subject of many a fascinating volume, since Sprengel's 

 Newly Discovered Secret of Nature (1793) ; it must suffice 

 here to notice that, so far as we can infer from the his- 

 tory half hidden in the rocks, the floral world must have 

 received a marked impulse when bees and other flower- 

 visiting insects appeared ; that for the successful propa- 

 gation of flowering plants it is advantageous that pollen 

 should be carried from one individual to another, in 

 other words, that cross-fertilisation should be effected ; 

 and that, for the great majority of flowering plants, this 

 is done through the agency of insects. How plants 

 became bright in colour, fragrant in scent, rich in nectar, 

 we cannot here discuss ; the fact that they are so is 



