Photo by J. M. Schreck 



A BLACK TERN ON ITS NEST (SEE PAGE 697) 



Its nest, of reeds and grasses rather closely woven, is found in grassy marshes or in 

 vegetation floating in a slough. It is an abundant species in the interior of the United States 

 and subsists chiefly on dragon flies and various aquatic insects. 



from mammals on the one hand and 

 from reptiles on the other are more ap- 

 parent than real. Thus flight, the most 

 striking of a bird's gifts, is shared by 

 bats among mammals. Egg-laying is the 

 habit of most reptiles and of three mam- 

 mals (the Australian duckbill and the 

 echidnas). But incubation by one or 

 both of the parents is peculiar to birds, 

 though the python is said to coil on its 

 eggs. 



Birds breathe more rapidly than either 

 mammals or reptiles, and their pneuma- 

 ticity, or power of inflating numerous 

 air-sacs and even certain bones, is unique. 



The temperature of birds ranges from 

 100° to 1 12°, while in mammals it reaches 

 98° to 100°, and in the comparatively 

 cold-blooded reptiles it averages only 40°. 



The skull in mammals articulates with 

 the last vertebra (atlas) by two condyles 

 or balls; in birds and reptiles by only 

 one. In mammals and birds the heart 

 has four chambers ; in reptiles it has but 

 three. 



BIRDS ARE DESCENDED EROM REPTIEES 



Mammals and reptiles both have teeth. 

 a_ character possessed by no existing 

 birds; but fossil birds apparently prove 



that early in the development of the class 

 all birds had teeth. 



Thus we might continue the compari- 

 son, finding that birds have no universal 

 peculiarities of structure which are not 

 present in some degree in either mam- 

 mals or reptiles, until we come to their 

 external covering. The reptile is scaled, 

 and so is the fish ; the mammal is haired, 

 and so are some insects ; but birds alone 

 possess feathers. They are worn by every 

 bird — a fit clothing for a body, which is 

 a marvelous combination of beauty, light- 

 ness, and strength. 



There is good evidence for the belief 

 that birds have descended from reptilian 

 ancestors. This evidence consists of the 

 remains of fossil birds, some of which 

 show marked reptilian characters and, as 

 just said, are toothed. It is unnecessary 

 to discuss here the relationship of the 

 bird-like reptiles, but, as the most con- 

 vincing argument in support of the theory 

 of the reptilian descent of birds, I men- 

 tion a restoration of the Archjeopteryx, 

 the earliest known progenitor of the class 

 Aves. This restoration is based on an 

 examination of previous restorations in 

 connection with a study of the excellent 

 plates which have been published of the 



702 



