INTRODUCTION xliii 



the weight of the bird during flight; but this is now 

 known to be an incorrect view. They serve to assist 

 respiration, and also, probably, to regulate the tem- 

 perature of the body. 



Concerning Eggs. — Birds, like their cousins the 

 reptiles, and certain low forms of mammals, lay eggs. 

 In birds these are always invested in a hard shell, 

 which is commonly coloured, often very beautifully. 

 How this colouring is formed, and how deposited on 

 the shell, we do not yet know, or at least only in part. 



The number of eggs laid by different species of 

 birds varies greatly. Thus some species lay but one 

 egg, as the Guillemot; some two, as the Pigeon. 

 Plovers lay four, Game-birds from twelve to twenty. 



When first formed the egg of a bird is extremely 

 small, but during its development there is added to 

 the germ, which is to grow into the chick, a great quan- 

 tity of yolk, to serve as food for the developing bird, 

 and outside this food-yolk there is deposited a quan- 

 tity of "white" of egg, and, finally, the whole mass 

 is enclosed within the shell. 



Although certain stages of development are passed 

 before the egg is laid, this development is soon sus- 

 pended, and is only resumed as a consequence of the 

 heat generated by the body of the brooding bird. 



When hatched, the young birds are in many species 

 extremely active, following their parents and finding 

 food for themselves soon after leaving the shell; in 

 other species the young leave the shell in a singularly 

 helpless condition — blind and naked — and these have 

 to be very carefully attended by the parents until they 

 are strong enough to fly and take care of themselves. 



