The flower constancy of the honey-bee was first accurately described by Aristotle (2) in the following terms: "On each expedition the bee does not fly from a flower of one kind to a flower of another, but flies from one violet, say, to another violet, and never meddles with another flower until it has got back to the hive". The phenomenon was well known to the 19th century floral biologists, Charles Darwin, Herman Muller and their colleagues (5, 6, 18, 21, 37, 53, 54). It was subsequently investigated by several students of insect behavior at the turn of the century, particularly by Felix Plateau, Gaston Bonnier, August Forel and Karl von Frisch (9, 26, 27, 29, 61). More recently the bearing of bee constancy on certain practical aspects of agriculture, such as problems of orchard management (10, 11, 13, 15, 16, 51), production of forage crops (40) and contamination of seed crops (4, 20), has come to the fore. The flower-visiting behavior of the honey-bee, which is a domesticated animal, has thus been the subject of numerous studies during this century and the last. One of the purposes of the following review is to summarize this scattered information for botanists. The flower-visiting habits of wild bees are not so well known as those of the honey-bee. An attempt has nevertheless been made to summarize the principal published observations on flower constancy in wild bees. These observations indicate that wild bees possess an instinct of flower constancy similar in general to that of honeybees. The writer will undertake, finally, to consider some of the possible consequences of the phenomenon of flower constancy in the evolution of the angiosperms.