Le Crime_, 193.

Let us now consider the criminal's physiognomy. In this connection it
must be borne in mind that a prolonged period of imprisonment will
change the face of any man, whether he is a criminal or not. Political
offenders who have undergone a sentence of penal servitude, and who may
be men of the highest character, acquire the prison look and never
altogether get rid of it. If a man spends a certain number of years
sharing the life, the food, the occupations of five or six hundred
other men, if he mixes with them and with no one else, he will
inevitably come to resemble them in face and feature. A remarkable
illustration of this fact has recently been brought to light by the
Photographic Society of Geneva. "From photographs of seventy-eight old
couples, and of as many adult brothers and sisters, it was found that
twenty-four of the former resembled each other much more strongly than
as many of the latter who were thought most like one another."[40] It
would, therefore, 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