By using the continuation theorem of coincidence degree theory, the existence of a positive periodic solution for the Lotka-Volterra population model, considered by Song-Chen. , is established.
"Periodic solutions for nonautonomous predator-prey system with diffusion and time delay." Hiroshima Math. J. 31 (3) 371 - 381, November 2001. https://doi.org/10.32917/hmj/1151105725