4 THE BIOLOGY OF BIRDS 



septum completed, and it is completed in crocodilian 

 reptiles. But although the crocodile has a four-chambered 

 heart, it does not, so to speak, make the best of it, for the 

 blood that goes to the posterior body is mixed blood (the 

 dorsal aorta being formed by the union of a right aortic 

 arch with pure blood, and a left aortic arch with impure 

 blood). In birds the aortic arch which gives off the blood- 

 vessels to the head, and is turned round posteriorly to form 

 the dorsal aorta, contains only pure blood. This must have 

 meant a great improvement in vigour of life — the supplying 

 of the body with pure blood only, and the complete separa- 

 tion of pure and impure blood in the heart. It is not 

 suggested, of course, that the primitive birds had a heart 

 as well equipped as that of modern forms, for the heart has 

 doubtless been subject to progressive improvement for 

 ages. Our suggestion is that antecedent to flight there was 

 an improvement in the circulatory system which made 

 attempts in the direction of flight more probable. 



§ 4. Warm-bloodedness 



Another pre-condition of the bird's success was warm- 

 bloodedness, that is, the power of regulating the temperature 

 of the body so that an approximate constancy is sustained. 

 Heat is produced in the oxidations or combustions that go 

 on in the body, chiefly in connection with the muscles, and 

 this heat, if not excessive, facilitates chemical reactions. 

 If there is a very low temperature in the outer world, the 

 body of the cold-blooded animal loses too much heat, and 

 the vital reactions are slowed down. In the warm-blooded 

 mammal in a very cold environment the loss of heat can 

 be lessened by constricting the blood-vessels in the skin, 

 or it can be counteracted by producing more heat internally. 

 The same is probably true of birds, but they have not been 

 much studied in this connection. When the warm-blooded 

 animal is in a very warm place and has itself an internal 

 rise of temperature, the danger of this may be counteracted 



