"It is harder to change human nature than to change rivers and mountains".

          Sometimes it is harder to change human's nature, than rivers and mountains. However, rivers and mountains are also considered as nature in its wider sense and that nature appears to be the various and complicated system on the planet, called the Earth. Which is known as a habitat of a Man. So the say, that it is easier to change outside than inside. 

Now let's stand back from the discussion and try looking into the history. Can you say, that humans character is similar to the one, that was thousand years ago? If it is too much, take half a thousand. Are the way of acting, goals and values of a man nowadays the same, that a man in twenties century had? Since the time, human being inherited the Earth, he has been changing the world for his comfort. And definitely has been changing himself, according to the conditions he made.  

A person truly can adapt to any circumstances. And that is good, because many of them are out of our will. Life often limits people's choice, and circumstances affect people's views. That's why after a great changes of our lives, we also change a lot.