NATURAL HISTORY 



I. BIRDS 



IN previous chapters of this book we have discussed the evolu- 

 tion of animals in general, the inclined plane of behaviour 

 and the everyday life of the body, and it has been necessary 

 to make many references to birds. But there are good reasons for 

 devoting a special chapter to this great class. Birds have entered 

 closely into human life, and in manifold ways. They supply food, 

 and they are the poet's symbols. Their feathers keep us warm 

 at night, and wing the arrow of the bowman. Birds save the 

 world from the continual menace of prolific insects, and they 

 gave the priests a basis for their auguries. To birds we must 

 trace the enormous nitrate beds of Chili which have fertilised the 

 soil of half the world, and we may thank them too for a share in 

 the impulse that has led man to his mastery of the air. Moreover, 

 most birds are joys for ever. Biologically regarded, birds are of 

 supreme interest in their solution of the problem of flight so 

 different from that of insects, Pterodactyls, and bats; in their 

 variability and plasticity within a comparatively narrow range; 

 and in their fascinating behaviour with its remarkable blending 

 of instinctive and intelligent activities. 



1 



Millions of years ago the evolution of birds from a reptilian 

 stock began, as has been already described in an early chapter of 

 this work. At first sight it is not easy to see any resemblance 



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