A case of co-evolution

This article said that species evolve together, and in competition. It says that as a species becomes better at escaping or resisting predators, those predators also develop new skills and evolve to be better suited to catch their prey. As this occurs those "prey species," continue evolving to avoid these predators. This goes back to that struggle for existence that occurs in the environment. They used the example of water fleas and the parasites that infest them, every time the parasite evolved new ways of infesting and harming the fleas, the fleas matched the parasite with a new way to resist the parasites. I agree with this theory because if predators didn't evolve with their prey, then we would have overpopulation of species as their predators would not be able to keep a equal level.