I noted particularly that these variations bore no necessary relation to each other, so that a large temporal muscle and zygomatic aperture might exist either with a large or a small cranium; and thus was explained the curious difference between the single-crested and the double-crested skulls, which had been supposed to characterise distinct species.
— from Darwinism (1889) An exposition of the theory of natural selection, with some of its applications by Alfred Russel Wallace