Darwin, who was naturally anxious to bring the facts of expression under his formula of evolution, began to collect data as early as 1838; and with characteristic thoroughness he went to all available sources,—to animals, to the human infant, to the insane, to works of art, to the play of the facial muscles under the electric current, to the different races of mankind. In his book of 1872 he distinguishes three main principles of expression; the titles will be understood from the examples. The first principle is that of _serviceable associated habits_. We all jump when we are startled, and wince when we are threatened; and the jump and wince of man are weakened survivals of the frightened animal’s leap out of danger, and of its cowering self-effacement in presence of a stronger enemy. The face of scorn, “curving a contumelious lip,” lays bare the canine teeth, as if for actual attack; the sneer of man is but a weakened survival of the snarl by which our stronger-jawed ancestors unfleshed their teeth for the combat. The second principle is that of _antithesis_. If indignation shows itself (according to the first principle) by squared shoulders and out-thrown chest, the opposite of this aggressive indignation, humiliation or self-abasement, shows itself in the opposed attitude of raised shoulders and indrawn chest, Shylock’s “patient shrug.” The third principle, lastly, is that of the _direct action of the nervous system_. Thus we all tremble from fear; and trembling is of no service, often of much disservice, and cannot have been at first acquired through the will, and then rendered habitual in association with any emotion; it must be directly due to the constitution of the nervous system.