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 evidence. In this time, the dog has developed into hundreds of breeds with a great degree of (36) . For example, heights at the (37) 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 patterns.
Dogs are highly social animals and this similarity in their (39) behavioral system accounts for their trainability, playfulness, and ability to fit into human (40) and social situations. This has earned dogs a unique position in the (41) of interspecies relationships. The loyalty and (42) that dogs demonstrate as a part of their natural instincts are (43) to be reflections of the human idea of love and friendship, leading many dog owners to view their pets as full-fledged family members. (44) . Dogs play a variety of roles in society and are often trained as working dogs. (45) . In many countries, the most common and perhaps most important role of dogs is as a companion. (46) .