Lettuce has been around for centuries. According to Wikipedia, the Egyptians first cultivated lettuce as edible leaves as early as 2680BC. It then made its way to Greece and Italy where it flourished.
Today, we see four main varieties of the species, Lactuca sativa, in our backyard or school garden. These include:
~ Crisphead or Iceberg,
~ Butterhead, Boston or Bibb (which is also a heading lettuce)
~ Romaine or Cos and
~ Looseleaf or Leaf Lettuce
Since they are all varieties of the same species they will cross-pollinate one another.