18 
SECOND BOOK. 
6. “ Again, there are some mammals — the bats — 
whose limbs fit them for flight in the air. 
“ Thus we see that the particular form of an 
animal is exactly suited to the kind of life it leads.” 
7. Mr. Johnson then led the boys close to the 
skeletons of a monkey, a dog, and a rat. 
“We can clearly see that the bones correspond 
in these three skeletons,” he said. “ There are the 
backbone and the ribs in each ; and you may notice 
that the limbs of all three animals are built on the 
same plan. 
8. “ The ribs form a kind of box to hold the heart 
and lungs, which are thus well protected. Every 
time we breathe we take air into our lungs, where 
tiny blood-vessels are spread out in a way that 
allows the oxygen of the air to get into our blood.” 
9. “Do all the backboned animals have lungs?” 
asked one of the boys. 
“ No, the fishes have not,” replied their friend. 
“ Mammals agree with each other in having lungs 
and in being air-breathers. They agree, too, in 
having the heart divided into four chambers, as 
you may see in this picture.” 
10. “ There are two chambers at the top and two 
below in the picture,” said William. “ Is every 
animal’s heart like that?” 
“Oh, no!” replied Mr. Johnson. “That is just 
what I want you to understand in learning how 
mammals agree among themselves, while differing 
