1. Feeny, P. P., Ecology51 (1970) 565.
2. Rosenthal, G. A., and Janzen, D. H., (eds), Herbivores. Their interaction with secondary plant metabolites. Academic Press, New York 1979.
3. Ehrlich, P. R., and Raven, P. H., Evolution18 (1965) 586.
4. Haukioja, E., Niemelä, P., Iso-Iivari, L., Ojala, H., and Aro, E.-M., Rep. Kevo Subarctic Rs. Stat.14 (1978) 5.
5. Whitham, T. G., in: Habitat and geographic variation. Eds R. F. Denno and H. Dingle. Springer, New York 1981.