xxii INTRODUCTION ^ 



It is, indeed, a question for the physiologist, rather 

 than for the ornithologist. 



It may seem that this attempt to answer the ques- 

 tion, "What is a bird?" has taken us rather far afield. 

 And on this account it may be well briefly to simi- 

 marise the facts which have been gleaned on the jour- 

 ney. 



In few words, then, a bird is a warm-blooded, egg- 

 laying feathered biped, having the fore-limbs modified 

 to form wings, and the hip-girdle so adapted as to 

 bring the hind-limbs far forward, to balance the body 

 in walking on them alone. These characters, there 

 can be no reasonable doubt, have gradually come into 

 being by the slow transformations in a long chain of 

 creatures, which, as we trace them back, are less and 

 less bird-like, and more and more like reptiles. 

 Though many links in this chain are yet missing, some 

 day they will almost surely be found. 



The evidence for this reptilian descent is abundant. 

 Every bird, in the course of its growth from the e.gg, 

 passes through more or fewer of the ancestral stages ; 

 and while some of these carry us back to phases of de- 

 velopment which belong to ancient types of birds long 

 since extinct, others carry us yet further, and show 

 that modern birds and reptiles have descended from a 

 common stock. 



Feathers. — Though feathers are so common, few 

 realise what marvels of structural beauty they are. 

 Nor is the peculiar fashion of their distribution over 

 the body even now generally recognised. Those who 

 have spent their lives in the study of living birds need 

 not be told that the feathers of a bird are not, as a 



