The study of butterfly migration was a passion for C. B. Williams. He made uncountable observations himself during his years in the Tropics and he had colleagues all over the world making new observations. He analysed and published the results in a long series of publications and became a world-leading authority on the subject. Through his research, he was able to shed light on many of the problems, which he had first formulated in his 1930 thesis. He published a much enhanced account on the subject in 1958.
