140 PROBLEMS OF BIRD LIFE. 



For see how it is with men. The blacksmith is not like the 

 soldier, nor the bicycler like the tailor. They use different 

 sets of muscles, and the men are unlike in size, shape, and 

 accomplishments. The blacksmith can bend an iron bar, but 

 he could not catch a runner in a race ; and neither he nor the 

 runner could make his fingers fly like the slender white fin- 

 gers of the musician. If blacksmiths' sons were always black- 

 smiths and musicians' children musical, we might expect much 

 more remarkable differences. 



In like manner, a bird's work changes its shape and struc- 

 ture ; eating one kind of food, using one form of exercise, the 

 bird, like the man, grows better and better fitted to follow his 

 own trade and more unfitted to take up any .other. We say 

 he becomes adapted to his kind of life, and that his structure 

 is modified (that is, changed) by his search for food. 



This is the first problem of bird life to find food. This is 

 the principle, The search for food results in modifications of 

 structure. Take this principle and see how it will explain the 

 shape of many kinds of birds that you see. Why is the swal- 

 low so swift and light of wing ? He hunts little dancing, flit- 

 ting flies. Why is the humming-bird so slender-billed and 

 quick-winged ? He seeks his insects out of the long tubes of 

 flowers as he poises buzzing before the blossoms. , Why is the 

 yellow warbler so trim and dainty ? He, too, eats insects, 

 but such as he finds in his pathway as he trips along the 

 branches, and so he needs neither strong wings nor long prob- 

 ing bill. All three feed .on insects, but they find them in 

 different places and hunt for them in different ways. The 

 way they find their food not the kind of food itself 

 decides what the structure of the bird will be. 



You may be able to discover for yourselves why the heron, 

 the loon, and the sea gull, which all live on fish, are yet so 



