She laughed. “Taxonomy! You can’t escape your training.” And she asked me to tell her about it.

First, I said, there were the people who had a genuine and powerful fascination with animals. She was that way herself, I said; when she saw a bird flying, there was a look on her face… it was like she was seeing a miracle.

She wasn’t so sure she approved of that; you have to be scientifically detached, you know. But she admitted the type certainly existed.

Then, I said, there were the stalkers. These people liked to crawl around in the bush tailing other creatures, like kids playing a game. I went on to explain why I thought this was such a powerful urge; it seemed to me that the life it led to was very similar to the lives led by our primitive ancestors, for a million long years. Living in camps, stalking animals in the woods: to get back to that style of life is a powerfully satisfying feeling.

Sarah agreed, and pointed out that it was also true that nowadays when you got sick of camp life you could go out and sit in a hot bath drinking brandy and listening to Beethoven, as she put it.

“That’s right!” I said. “And even in camp there’s quite a night life, you’ve all got your Dostoevski and your arguments over E.O. Wilson… it’s the best of both worlds. Yeah, I think most of you are stalkers on some level.”

“But you always say, ‘you people,’ ” Sarah pointed out to me. “Why are you outside it, Nathan? Why did you quit?”

And here it got serious; for a few years we had been on the same path, and now we weren’t, because I had left it. I thought carefully about how to explain myself. “Maybe it’s because of type three, the theorists. Because we must remember that animal behavior is a Very Respectable Academic Field! It has to have its intellectual justification, you can’t just go into the academic senate and say, ‘Distinguished colleagues, we do it because we like the way birds fly, and it’s fun to crawl in the bushes!’ ”