What is the topic of the passage?

What will man be like in the future in 5,000 or even 50,000 years from now? We can only make a guess, of course. For man is slowly changing all the time. Let us take an obvious example. Man, even five hundred years ago, was shorter than he is today. Now, on average, man is about three inches taller. Five hundred years is a relatively short period of time. Again, in the modern world we use our brains a great deal. Even so, as time goes on, however, we shall have to use our brains more and more and we shall need larger ones ! This is likely to happen too: the head, in particular the forehead, will grow large. Nowadays our eyes are in constant use. In fact, we use them so much that very often they become weaker and we have to wear glasses. But over a long period of time, on the other hand, they, as a result, re likely to grow weaker. At the same time, however, our fingers will grow more because they are used a great deal in modern life.