Especially for large mammals with long lifespans and a longer period of time from one generation to the next, the accumulation of change is a slow process. It takes many tens of thousands of years for them to cast off the form of their ancestors. Even for smaller animals the process may take ten thousand years or more.
尤其高齡、大型的哺乳動物，代代相傳時間更長，遺傳變異累積緩慢，需要幾十萬年光陰才可能擺脫老祖先面貌，小型野生動物也要上萬年。