In Müllerian mimicry an advantage is gained when unpalatable species resemble each other, especially when the predator has a good memory for colour (as birds, for instance, do have). Thus one trial may work to dissuade a bird from several species of butterfly which all fly the same 'flag'. Brazilian butterflies provide some of the most extraordinary examples of mimicry, and Müller, Bates and Wallace all had lengthy experience of this. All three traveller-naturalists believed firmly that such systems of mimicry could only come about by means of natural selection, and all of them wrote about it.