3. Passage Twenty-One What will man be like in the future—in 5，000 or even 50，000 years from now? We can only make guesses， of course， but we can be sure that he will be different from what he is today. 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， men are about three inches taller. Five hundred years is relatively short period of time， so we may assume that man will continue to grow taller. Again， in the modern world we use our brains a great deal. Even so， we still make use of only about 20% of the brain’s capacity. As time goes on， however， we shall have to use our brains more and more， and eventually we shall need larger ones. This is likely to bring about a physical change to the head， in particular the forehead will grow larger. 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 very long period of time it is likely that man’s eyes will grow stronger. On the other hand， we tend to make less use of our arms and legs. These， as a result， are likely to grow weaker. At the same time， however， our fingers will grow more sensitive because they are used a great deal in modern life. But what about hair? It will probably disappear from the body altogether in course of time because it does not serve a useful purpose any longer. In the future， then， both sexes are likely to be bald. Perhaps all this gives the impression that future man will not be a very attractive creature to look at. This may well be true. All the same， in spite of all these changes， future man will still have a lot in common with us. He will still be a human being， with thoughts and emotions similar to our own.Man’s forehead will grow larger because ( ) .