2 PRACTICAL ZOOLOGY 



the way we do, and it is true that most of those we encounter 

 in our daily lives are terrestrial in habit; but we should remem- 

 ber that human beings, miners, for instance, may live under- 

 ground for long periods of time, or may remain in the water for 

 hours without injury, or may even move through space in a 

 balloon or aeroplane. These departures from activities on the 

 earth's surface are, however, only temporary, and man's habitat 

 is to be considered purely terrestrial. 



Human beings share their terrestrial habitat with most four- 

 footed beasts, with the frogs and toads part of the time, and 

 with a host of the lower animals, such as insects, spiders, and 

 certain snails and worms. Fortunately, land animals do not all 

 try to live in the same sort of habitat, but are distributed over 

 the entire earth's surface. Some seem to prefer the cold polar 

 regions, others temperate or tropic zones; some inhabit the 

 open plains, others live on forested mountains; and even the 

 height above sea level has an influence upon the kind of animals 

 inhabiting any particular area. Animals that live on the sur- 

 face are better known than those that live in the ground, since 

 the latter are less often seen. Among these ground inhabitants 

 or subterrestrial animals are the earthworm, and many other 

 worms, certain insects, crayfishes, and spiders, some of the 

 snakes, a few birds like the burrowing owl, and a number of 

 quadrupeds, of which the pocket gopher, mole, woodchuck, and 

 prairie dog are common examples. Almost all of these animals 

 must come to the surface from time to time to get food and for 

 other purposes, but their true homes are in the ground. 



The animals of another group spend a part of their time flying 

 about in the air; to these the name aerial has been applied. The 

 most notable aerial animals are the birds, flies, butterflies, and 

 other insects, but there are a few flying quadrupeds, the bats, 

 and a few animals like the flying squirrels, flying lizards, and 

 flying fish, which do not really fly but only spread out the mem- 

 branes with which they are provided and sail through the air for 

 comparatively short distances. 



