“…For example, in [1-3, 12, 14-20], the authors studied existence of positive periodic solutions for population competition systems, in [4,5,8,13,21], the authors studied existence of positive periodic solutions for population cooperative systems, in [9], the authors studied existence of positive periodic solutions for population predator-prey system, and in [6,11], the authors studied existence of positive periodic solutions for single species systems. The continuation theorem is a powerful tool to study the existence of periodic solutions of periodic high-dimensional timedelayed problems.…”