I read a book by the American historian, Arthur Lovejoy, entitled The Great Chain of Being. And, in this book, Lovejoy shows how an idea, a very basic broad idea, that Aristotle had first developed, changed in the course of history and became more widespread, more dominant, and that idea, so to speak, had an evolution, just like a kind of animals would have an evolution through time. And this thought that an idea could have an evolution and that such an idea could have a rather dominant influence on the particular period in which it was changed, greatly fascinated me and has been, sort of, in the background of my thinking ever since in the numerous historical papers that I have written since that time. Now… I'm, of course, not the only one that has been thinking of such… along these lines and it has become quite clear to a number of historians that concepts, which are these ideas, play a great role in science. And I go even further: I say that new theories are found in the physical sciences usually on the basis of laws while new theories in biology are usually based on new concepts, like Darwin's concept of natural selection, like the concept of territory, like concepts like sexual selection or in ecology, like… resources or competition or predation. All these… none of these things are there at the basis of… of biological theories is based on a law. They are all based on concepts. So concepts are, in my opinion, the most important thing for the history of biology.