From Mastiff to Miniature Poodle, Dogs Know Each Other by Sight

Anyone who's walked a dog and seen it spring to attention when another dogs rounds a corner—even though that animal is still a full block away—may have wondered how exactly dogs recognize each other. What makes a golden retriever perk up its ears and wag its tail at an approaching greyhound but not, say, a stroller? Why does it ever occur to a dachshund to play with a pit bull in the park? Why don't average-sized dogs chase toy breeds away as if they were squirrels?