INTRODUCTION 47 



processionary caterpillars, that not only destroy whole 

 forests of oak, but also cause violent inflammation on the 

 human skin by their hairs. It has been proved that 

 most of the caterpillars eaten by the cuckoo were 

 already stabbed by ichneumon-flies. Thus they were 

 already full of larvae, which would have attacked 

 caterpillars in turn after they had crept out. These 

 were destroyed by the cuckoo ; and, as they would have 

 proved the most effective restriction on the plague of 

 caterpillars, we should regard the cuckoo rather as a 

 harmful than a useful bird. 



What do we find in the case of other insectivorous 

 birds ? 



Unfortunately, we have to class many of them as 

 mischievous which were generally regarded as useful. 

 The redstart only too often visits the hive for the 

 purpose of filling its stomach with the useful honey- 

 bearers. The fly-catcher destroys the caterpillar-flies 

 the bitter enemies, as the name indicates, of the 

 caterpillar. The kingfisher, of which anglers complain 

 so bitterly, feeds mainly on water-boatmen, the stinging 

 water-bugs that are very dangerous to the young fishes ; 

 it ought, therefore, in this respect to be considered 

 useful. 



But we need not delay any longer in determining 

 which animals are useful and which mischievous. I 

 believe that the harm done by the higher animals, 

 especially mammals and birds, is never serious enough 

 to justify us in making every effort to destroy them. 

 We have seen that in a biocoenosis all animals are 

 related to each other, and that the multiplication of one 



