CHARACTERS OF BIRDS. 
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The characters that distinguish birds from mamm: nels 
on the one hand, and from reptiles on the other, are n nore 
parent than real. Thus flight, the most striking « is 
bird’s gifts, is shared by bats among mammals. Egg-lay 
ing is the habit of most reptiles and of three mammals — 
(the Australian duckbill and the echidnas). But inew Re | 
tion by one or both of the parents is peculiar to b « | 
though the python is said to coil on its eggs. ih 
Birds breathe more rapidly than either. mammals or 
reptiles, and their pneumaticity, or power of inflatir g 
numerous air-sacs and even certain bones, is unique. 
The temperature of birds ranges from 100° to 112°, 
while in mammals it reaches 98° to 100°, and in the com- 
paratively cold-blooded reptiles it averages only 40°. | 
The skull in mammals articulates with the last vei 
bra (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. 
Mammals and reptiles both have teeth, a charac 
possessed by no existing bird; but fossil birds ¢ appar 
ently prove that early in the developmais of the ¢ class 
all birds had teeth. ay 
Thus we might continue the comparison, finding that 
birds have no universal peculiarities of structure » 
are not present in some degree in either mamma 
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 feathe 
They are worn by every bird—a fit clothing for a 
which is a marvelous combination of beauty, light: az 
and strength. a 
There is good evidence for the belief that birds I 
descended from reptilian ancestors. This evidence ¢ ce 
sists of the remains of fossil birds, some of which ¢ 
marked reptilian characters and, as just said, are toot 
