INTRODUCTORY 9 



we may count the fact that the heart of the bird, like that of the 

 mammal, is a four-chambered heart, whereby a more perfect oxi- 

 dation of the blood is possible than is the case with the reptiles, 

 wherein the heart has but three chambers. And for this reason, when 

 passed through a four-chambered heart, the impure blood brought 

 back from the body to the right side of the heart is driven thence to 

 the lungs to be thoroughly purified by the air drawn in during 

 breathing and returned to the left side of the heart, whence it is sent 

 over the body without any mingling of the two streams — pure and im- 

 pure. In the reptile this mingling is unavoidable, and consequently 

 a smaller quantity of the heat-giving oxygen is brought into the 

 system. But, strangely enough, the blood of the reptiles and birds 

 agrees in this, that the little red bodies, or " corpuscles," whose duty 

 is to absorb the air from the lungs and the carbonic acid from the 

 tissues of the body, have each a central " kernel," or '' nucleus," 

 whereas the blood corpuscles of the mammal have no nucleus. 



It would be wearisome to further expand this question of the 

 circulation of the blood, since to thoroughly understand this matter 

 a somewhat intimate knowledge of physiology and chemistry is re- 

 quired. It is, in short, a question for the physiologist and the medical 

 man, rather than for the ornithologist. 



It may seem that this answer to our question, " What is a bird ? " 

 has taken us rather far afield. And on this account it may be well 

 briefly to summarise the facts which have been gleaned on the 

 journey. 



In a 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. These characters, there can be no 

 reasonable doubt, have gradually come into being by the slow trans- 

 formation of a long chain of creatures which, as we trace them back, 

 grow 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 egg onwards, passes through 

 more or fewer of the ancestral stages ; and while some of these carry 



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