What Is a Mammal? 
When visitors at the zoo or the natural history museum ask to 
see the "animals," they almost always mean the mammals. If 
they want to see birds, snakes, crocodiles, or insects, they say so. 
Mammals, whether the visitors realize it or not, are those ani- 
mals that feed their young with milk. The word comes from the 
Latin, mamma, meaning "a nipple." 
Besides employing this basic method of rearing their young, 
most mammals give birth to their babies alive, have four legs, 
a more or less complete coat of fur, and a number of unique 
features in their internal anatomy. Exceptions to the foregoing 
include the Duckbill that lays eggs, the Monkeys that have hands 
instead of paws, the Whales, Dugongs, and Seals that have 
more or less converted their limbs into paddles, the Bats that 
have their hands made into wings, the Pangolins that develop 
large scales mixed with only a few hairs, and the Dugongs and 
Whales that have almost completely ceased to grow a hairy 
covering but have developed instead a dense layer of insulating 
fat beneath their skins. All these exceptions conform to the basic 
fact stated above : they give milk tQ their young. 
SPECIAL ADAPTATIONS IN MAMMALS 
The nearest relatives of the mammals are the reptiles, the 
birds, and the amphibia (frogs and salamanders). All have the 
same type of skeletal system — a framework of bones centered 
