A vast literature exists upon this subject also. Darwin’s “Expression
of the Emotions” is, of course, classical. Lindsay’s “Mind in the Lower
Animals” (2 vols.), and Mills’s “Animal Intelligence” are useful books.
Some original and ingenious speculations are contained in Ouspensky’s
“Tertium Organum.” A most interesting work, from a practical point
of view (that of a trapper and hunter) is “How Animals Talk,” by
William J. Long. A study of the so-called “talking animals” is “Lola:
a Contribution to the Thought and Speech of Animals.” Chapters on the
marvellous horses of Elberfeld are contained in Maeterlinck’s “Our
Unknown Guest,” and in my own “Modern Psychical Phenomena.”
The question of _Instinct_ in animals has also received extensive
treatment. C. Lloyd Morgan’s works are classical in this respect:
“Habit and Instinct,” “Instinct and Experience,” etc. See also
Chadbourne’s “Instinct in Animals and Men,” and McDougall’s treatment
of the subject in his “Social Psychology,” in his “Psychology,” and
a number of articles on this subject in the “Journal of Abnormal
Psychology,” and similar periodicals.