1. Dunlap, K., and Mowrer, O. H., J. comp. Psych., 11, 99–113 (1930).
2. Fremlin, J., New Scient., 56, 26–28 (1972).
3. Kare, M. R., in Avian Physiology (edit. by Sturkie, P. D.), 406–418 (Cornell University Press, 1965).
4. Walls, G. L., The Vertebrate Eye and its Adaptive Radiation (Cranbrook Institute, Bloomfield Hills, Michigan, 1942).
5. Daanje, A., Behaviour, 3, 48–98 (1951).