Yes, yes I did my homework - I googled this and got many results but none seem to answer my query. I am not a historian so bear with me. I had a picture in my mind that civilizations seem to appear after we discovered agriculture and thereafter each civilization advanced and continued to transform till today. But it seems that civilizations do appear but get destroyed after sometime. They do not continue forever. I am confused as we do not see this today right? In modern history all the empires/countries are stable, right? Why is this behavior?