seem that the action of unconscious imitation, arising from constant contact, is capable of producing a remarkable change in the features, the acquired expression frequently tending to obliterate inherited family resemblances.  According to Piderit, physiognomy is to be considered as a mimetic expression which has become habitual.  The criminal type of face, so conspicuous in old offenders, is in many cases merely a prison type; it is not congenital; men who do not originally have it almost always acquire it after a prolonged period of penal servitude.

    [40] Daily News, June 12, 1890.

But apart from the prison type of countenance, it is highly probable that a distinct criminal type also exists.  Certain professions generate distinctive castes of feature, as, for instance, the Army and the Church.  This distinctiveness is not confined to features alone, it diffuses itself over the whole man; it is observable in manner, in gesture, in bearing, in demeanour, and is constantly breaking out in a variety of unexpected ways.  In like manner the habitual criminal acquires the habits of his class.  Crime is his profession; it is also the profession of all his associates.  The constant practice of this profession results in the acquisition of a certain demeanour, a certain aspect, gait, and general appearance, in many instances too subtle to define, but, at the same time, plain and palpable to an expert.

The slang of criminals is also explicable on the same principle.  Every trade and calling has its technical terms.  The meaning of these terms is hidden from the rest of the world, but the origin of their existence is not difficult to explain.  The jargon of the criminal arises from the same causes and is constructed on exactly the same principles as the technical words and phrases of the man of science.  When a man of science is compelled to make frequent use of a phrase, he generally gets rid of it by inventing some technical word; it is precisely the same with criminals.  With them technical words are used instead of phrases, and short words instead of long ones in all matters where criminal interests are intimately concerned, and on all topics which are habitually the subjects of conversation among the criminal classes.  The language of the Stock Exchange with its Bulls, Bears, Contangos, and other short and comprehensive expressions for various kinds of stocks, is on all fours with the slang of criminals, and it is not necessary to resort to atavism in order to explain it.  It arises to supply professional needs, and criminal argot springs up from exactly the same cause.