30 [ The Origin of Species, London, 1875, p. 63.]
31 [ "There is only one aim in all generalization—the finding of signs that are fit to be trusted, so that, given one fact, another may be inferred."—A. Sidgwick, The Process of Argument, London, 1893, p. 108.]
32 [ W. James, Psychology, New York, 1890, Vol. II, p. 342.]