My 1942 book Systematics and the Origin of Species continued to sell and I was repeatedly asked to bring out a revised edition. However, I had the feeling that so much had happened since the original publication, and furthermore, by taking out all the material on the methods and principles of systematics and publishing it as a separate volume, it seemed the volume no longer should be republished. And I told Columbia [University Press] to stop printing it and let it go out of print. However, there was, of course, a need for a replacement volume which would be rather different and I worked on it very hard, and I still have in my office, and I don't quite know what to do with it, four versions of this book. Whenever I finished one, I would go back to chapter one, revise that again, chapter two, revise it… by the time I had revised the last chapter I felt there was a great need to revise again chapter one. And I finally said, well, this can go on until Doomsday, and I decided now I publish it. And that was in 1963, and the new book was called Animal Species and Evolution.