All species of animals, including man, of course, have developed very gradually over millions and millions of years. In this way, each animal strengthens and improves those parts of its body which are particularly useful, and the various parts which are not so useful diminish or are adapted. This evolution happens very, very slowly, and changes can only be seen over numerous generations. In four-legged animals, or quadrupeds as they are called, the tail is very important for balance, and in many cases it is used almost as another limb, for instance when monkeys swing through the trees. Man, however, walks on two legs – he is called a biped – and he does not need the tail for balance. As a matter of fact, a small very rudimentary ‘tail’ can be found at the base of our spines, but it does not help us at all with balance or movement.