In 1866, he wrote a paper entitled "Observations on an Ethnic Classification of Idiots" in which he put forward the theory that it was possible to classify different types of conditions by ethnic characteristics. He listed several types including the Malay, Caucasian and Ethiopian types. In the main, the paper is about what is known as Down syndrome, named after him, but which he classified as the Mongolian type of idiot. As a result, Down syndrome was also known as "Mongolism" and people with Down syndrome referred to as "Mongoloids". Down's paper also argued that if mere disease is able to break down racial barriers to the point of causing the facial features of the offspring of whites to resemble those of another race, then racial differences must be the result of variation, affirming therefore the unity of the human species. Down used this reasoning to argue against a tendency he perceived in his day to regard different races as separate species.