填空题

Dogs were first domesticated from wolves at least 17,000 years ago, but perhaps as early as 150,000 years ago based upon recent genetic fossil and DNA (36) . In this time, the dog has developed into hundreds of breeds with a great degree of (37) . For example, heights at the shoulder range from just a few inches to roughly three feet, and colors range from white to black, with reds, grays, and browns (38) in tremendous (39) . Dogs are highly social animals and this (40) in their overall behavioral system (41) for their trainability, playfulness, and ability to fit into human (42) and social situations. This has earned dogs a (43) position in the realm of interspecies relationships. (44) are thought to be reflections of the human idea of love and friendship, leading many dog owners to view their pets as full fledged family members. Dogs seem to view their human companions as members of their pack, and make few, if any, distinctions between their owners and fellow canines. (45) . For dogs that do not have traditional jobs, a wide range of dog sports provide the opportunity to exhibit their natural dog skills. In many countries, the most common and perhaps most important role of dogs is as companions. Dogs have lived with and worked with humans in so many roles that (46) .