It's impossible to visualize all the varieties of the tomato. I like to compare it to dogs, of which there are only about 400 different breeds. Tomatoes, however, have thousands of varieties, all genetically distinct. A Mortgage Lister beefsteak and a Sara's Galapagos are about as different as a dachshund and a Great Dane. They each have different characteristics, tastes, disease and pest resistance, and culinary uses. Every year, the folks at Seed Savers Exchange, an organization of gardeners dedicated to preserving heirloom crops, grow hundreds of varieties at Heritage Farm and offer them up to the public to see and sample.