听力原文：W: Professor Marnes, I wonder if you can fill me in on your lecture last Friday. I had to attend a scholarship award ceremony. M: Oh well, congratulations. I hope you were rewarded handsomely! W: Well, every bit helps. So, about your lecture, I understand you were talking about extinctions. M: Yes. Well, the crux of my talk was just that we tend to think of extinction as a dramatic event, but most species die out over quite a period of time. W: Why do they die off? I thought they were continuously improving themselves. Natural selection, I think you once mentioned. M: Ah, but you see while there is natural competition between the species, what determines which species survive is largely by chance. W: I don't get it. Why do species bother competing? M: Well, there are short-term advantages. But many species also are helped by others. For example, the common housefly and cockroaches might have died off years ago if not for humans. W: But you're not saying that humans are so successful merely because of chance? M: To a certain extent, humans were initially lucky enough to have the right weather conditions and a lack of predators, but now, of course, we survive by ingenuity! W: So we may never become extinct. M: No, because we may be in a crash course to extinction by our continuous exploitation of the environment. We are a relatively young species and our time is not yet overdue. W: But there are 6 billion of us! M: Yes and there're many more houseflies too! Each with the capacity to spread one disease from one person to another in a fast period of time. (23)