My form master was also the biology teacher. He really regretted that I haven’t chosen medical university and not being interested in biology. However, he had a couple of things I learnt. One is the concept of rabbits and foxes. In a place where only rabbits and foxes are present, their population is only dependent on each other. Obviously, there are plants that rabbits can eat, but let’s assume plants and other life forms don’t influence the big picture. In the beginning, there are only a couple of rabbits and foxes. As rabbits breed, their number will increase, so foxes will have more food and their number will also increase. However, their increased number will result in decreasing number of rabbits, so they will have food shortage and will die at some point. Assuming neither rabbits, nor foxes will go extinct, their population will eventually reach a point of balance, where there is just enough number of rabbits to feed the foxes, but not enough to have their number increase. This state doesn’t need any external control to achieve and maintain, and it is analogous to many areas of the civilized world.