THE DOG The dog is one of the most popular pets in the world.It ordinarily remains loyal to a considerate master,and because of this the dog has been called man's best friend.Class distinctions between people have no part in a dog's life.It can be a faithful companion to either rich or poor.Dogs have been domesticated for most of human history and have thus endeared themselves to many over the years.Stories have been told about brave dogs that served admirably in war or that risked their lives to save persons in danger.When Pompeii--the Roman community destroyed by Mount Vesuvius in AD 79--was finally excavated,searchers found evidence of a dog lying across a child,apparently trying to protect the youngster.Perhaps few of the millions of dogs in the world may be so heroic,but they are still a source of genuine delight to their owners.A dog fits easily into family life.It thrives on praise and affection.When a master tells a dog that it is good,the animal happily wags its tail.But when a master scolds a dog,it skulks away with a sheepish look and with its tail tucked between its legs.People in the city as well as those in other areas can enjoy a dog.Medium-size or small dogs are best suited for the confines of the city.Large dogs need considerable exercise over a large area.Dogs are not always well thought of,however.In recent years dogs in the city have been in the center of controversy.Some people have criticized dog owners for allowing their pets to soil sidewalks and lawns,although in some cities laws oblige owners to walk their dogs along street curbs.In turn,dog owners have argued that the animals serve as protection against vandals and burglars and thus protect their detractors as well as their owners.When a person decides to own a dog,he should be prepared to care for it properly.For a dog to stay healthy it must be correctly fed and adequately groomed,and its medical needs must be met.For a dog to be well-mannered it must be properly trained.It should never be ill-treated or mishandled.Otherwise,it will bite in its own defense.The wild ancestors of all dogs were hunters.Wolves and other wild relatives of the dog still hunt in packs for their food.Dogs have retained the urge to be with the pack.This is why they do not like to be left alone for long.Some breeds of dogs still retain the hunting instinct.Dogs exist in a wide range of sizes,colors,and temperaments.Some,such as the Doberman pinscher and the German shepherd,serve as alert and aggressive watchdogs.Others,such as the beagle and the cocker spaniel,are playful family pets,even though they were bred for hunting.Still others,such as the collie and the Welsh corgi,can herd farm or range animals.Each of the dogs just mentioned is a purebred.A mongrel dog,however--one with many breeds in its background--can just as easily fit into family life.Dogs have been with humans since prehistoric times.Over the years they have performed various services.They have pulled sleds over snowy tracts.They have delivered messages,herded sheep and cattle,and even rescued persons trapped in the snow.Dogs have served as a source of food,too.The ancient Romans are said to have prized certain kinds of dog stew.The Aztecs of ancient Mexico raised tiny dogs,thought to be the forebears of the chihuahua,to feed the large carnivores in the private zoos of the Aztec rulers.In the past dogs have even been worshiped as gods.Recently,they have been used in drug research,medical experimentation,and space science.Soviet scientists launched dogs into space to test the ability of mammals to survive the rigors of space travel before people were sent up.Dogs are trained as guard dogs in peacetime by the United States Army and other military services.Because of their keen sense of smell,dogs are used by police at times to track down escaped prisoners.Law enforcement agencies also rely on the dog's acute sense of smell to uncover illegal drugs.And specially trained dogs serve as the "eyes" of the blind,guiding the steps of their sightless masters around obstacles and hazards.