In a limited space, limited resources and limited environment! Rats and people all have their own class struggles. When the class differentiation was completed, they began to give up breeding. Because this is just like human society, no matter how hard you try, you can't change it. Your own economic and social status! At first, the mice did not reproduce directly, but waited until 104 days later. Obviously, rats have reproductive needs! This is very similar to today's human society! The class division of human society has been completed, so most people are reluctant to have children and get married at first. And it's getting worse!