I did this work for my doctoral thesis under the supervision of Peter Neumann who was one of the most important figures in my mathematical development. Varieties of groups are classes defined by identical relations and, in the 1960s, it was a hot question whether there existed varieties which needed an infinite number of defining relations. Once this question was answered (there are!) the study of varieties somewhat died away. I'd like to say that I was so far-sighted a mathematician that I left the subject voluntarily - but that would be a lie.