Wolves prey on deer, and people want to kill them instead of killing them. They violated the principle of "survival of the fittest". As a result, there are fewer and fewer wolves. I thought there should be more deer, but the opposite is true. Why is this? Because the deer has no worries, only cares about the free life. It turns out that when the wolf chases the deer, the deer will run. Now that there is no wolf, the deer's physique will inevitably decline until it dies. It can be seen that the role of hardship is not only beneficial to people, society and the whole nature.