In his chapter on instinct, Charles Darwin acknowledges that instinct is one of the most difficult for him to explain through natural selection. However, through examples such as beehives and the cuckoo, Darwin creates a convincing case for the gradual evolution of instinctual behaviors. A well-known example of instinct in the United States is the migration of the monarch butterfly. These insects migrate to the southern United States and Mexico every winter, and as far as Canada in the summer. This process takes six to eight generations in total to complete, making it impossible for the path to be learned and passed on through teaching. For decades, how monarch butterflies find their way, using similar routes as their ancestors, has been a mystery. Recently, however, a breakthrough has been made in the discovery of photoreceptor proteins called cryptochromes. These proteins allow monarchs to see ultraviolet light, and, most interestingly, to sense the earth's magnetic field.