GROUPS OF ANIMALS 257 



ming ; mice, beavers, and squirrels have the front teeth 

 modified for gnawing ; bats have the fore limbs modified 

 for flying. 



Birds. This is one of the largest groups of animals. 

 It has boon estimated that there are about 10,000 species 



Copyright Detroit Photographic Co. 



Fig:. 113. Sea lions are modified for life in the water. Their 

 feet serV'C as swimming organs and their bodies are fish-like in 

 form. 



belonging to it. Yet. there is scarcely another group 

 of animals so easy to distinguish at sight as this. The 

 characteristics common to all birds are : the body 

 is covered with feathers ; the fore limbs are modified 

 to form wings: they all produce eggs; and the jaw is 

 always covered with a horny substance forming a bill. 

 Birds differ greatly in their habits of life and many 

 interesting adaptations occur in this group. Some birds 

 have the feet adapted for perching, others for swim- 



