In the 1930s a California publisher, Nature Games, produced a popular series of card games, based on the traditional card game Authors. This kind of matching or "families" game involves groups of four cards, each with a different title of a work by a single author. In earlier versions, the cards represent members of a single family. The Nature Games versions represent different individual specimens within a species of plant or animal. For example, four butterflies of the milkweed family. Players try to make matching sets of the four cards to score points. The Authors game and its many variants survive to contemporary times. These 1930s examples prove that subject variants were popular at that time.