Bates’s theory of mimicry did not account for the superficial resemblances between two or more unpalatable species. Müller explained such mimicry, known today as “Müllerian mimicry”. A predator must learn which potential prey are palatable. The coloration of an unpalatable species serves as warning colouration to predators. When warning colouration is shared by two or more unpalatable species, the warning colours are recognized more quickly by the predator and the number of individuals destroyed in each species is reduced while the predator learns. Müller's account contained one of the earliest uses of a mathematical argument in evolutionary ecology to show how powerful the effect of natural selection would be:

"Instead of a general deduction, which is by the way extremely simple, I give an example. There may in a certain area live two unpalatable species; 10,000 individuals of the first species, and 2000 of the second. The predators living in the same area may eat per year 1200 individuals of each [distinct] unpalatable species per year until they avoid it as such. Each species would lose this many if they appeared different; but if they are very similar so that experience with one species benefits the other, then the first species will lose 1000 and the second 200 individuals. The first species therefore will gain because of its similarity 200 individuals, or 2 % of the total number, the second will however gain 1000 individuals, which is 50% of the total number - from this consideration it follows further that probably in some cases (for example Thyridia and Ituna) the question which one of both species is the original and which one is the copy is an irrelevant question; each had an advantage from becoming similar to the other; they could have converged on each other" (http://www.ucl.ac.uk/taxome/jim/Mim/muller1878.html, accessed 03-2018).