My mentor was Professor Erwin Stresemann, who was at that time clearly the world's leading ornithologist, a man who was not only highly intelligent, but very hard working; he edited two journals, he wrote a big textbook, he supervised students, he sent out expeditions. I never could understand how he could handle all these things, and he greatly influenced me because his thinking in the field of systematics was really absolutely at the foremost frontier. Another colleague there... who was only about four years older than I, another assistant, was Professor [Bernhard] Rensch who very much influenced me because he wrote a book on speciation, on the origin of species. And even though I didn't agree with every detail, it made me think about all the problems involved there and much of what he said influenced me and I followed it. In fact, in some ways he influenced me more than Stresemann had because Stresemann and I didn't talk as much about such matters.