134 THE WORLD OF LIFE CHAP. 



tree, hop rapidly about it, and then perhaps fly off to 

 another, having apparently decided that the first one had 

 already been nearly exhausted. But in the few minutes of 

 their absence they are always able to fill their mouths with 

 small caterpillars, flies, grubs, etc., and return to the nest, 

 not only from morning to night on one day, but the same 

 day after day, for at least a fortnight and often much longer, 

 till their first brood is fully fledged and able to provide for 

 themselves. But unless the numbers of insects and their 

 larvae were enormous, and were increased day by day by 

 fresh hatchings from the egg as fast as they were devoured, 

 hosts of these young birds would perish of hunger and cold. 

 For if the parents had to range far away from their nests, 

 and could not find the necessary supply so quickly as they 

 do, the young birds would be subject to attack from some 

 of their numerous enemies, would suffer from cold or wet, 

 and as they grew older would often, in their frantic struggles 

 with each other, fall out of the nest and quickly perish. 



What wonderful perfection of the senses must there be 

 in these various parent birds ; what acuteness of vision or of 

 hearing ; what rapidity of motion, and what powerful instinct 

 of parental love, enabling them to keep up this high-pressure 

 search for food, and of watchfulness of their nests and 

 young, on the continuance of which, and its unfailing 

 success, the very existence of those young and the continu- 

 ance of the race depends. But all this perfect adaptation 

 in the parent birds would be of no avail unless the insect 

 tribes, on which alone most of them are obliged to depend, 

 were as varied, as abundant, and as omnipresent as they 

 actually are ; and also unless vegetation were so luxuriant 

 and abundant in its growth and so varied in its character, 

 that it can always supply ample food for the insects without 

 suffering any great or permanent injury to the individual 

 plants, much less to any of the species. 



By such considerations as these we learn that what we 

 call insect-pests, when they are a little more abundant than 

 usual in our gardens and orchards, do not exist for them- 

 selves alone as an apparently superfluous and otherwise 

 useless part of the great world of life, but are, and must 

 always have been throughout long past geological ages, 



