Originally published in 1931, The Intelligence of Animals sets out detailed studies of various birds and beasts, some in the wild, some under domestication, and some under semi-domestication, with a view to illustrating the supreme importance of the "mental factor" in biology. These studies in comparative psychology have been undertaken in the hope of obtaining light on the distribution, survival and extinction of species. The author endeavours to analyse the behaviour and mental characteristics of certain birds and beasts, with especial reference to the question of whether these attributes are useful, or a handicap to them, in the stress of life. Today it can be read in its historical context.