Who discovered the twin method?

The twin method is usually credited to Francis Galton's 1875 article on twins. However, Galton did not propose the comparison between identical and fraternal twin resemblance which is the essence of the twin method. Although the twin method was "in the air" in the mid-1920s, the first descriptions of the method appeared in an article by Curtis Merriman and in a book by Hermann Siemens, both in 1924, 50 years after Galton's paper.