_The classification of criminals_. The legal classification of criminals according to the nature of their crime is manifestly of no use for scientific purposes. What we need is a classification of criminals according to their own peculiar nature. Inasmuch as the nature and conduct of a criminal person is largely a matter of his psychology the most scientific classification of criminals must be upon a psychological basis; and a simple psychological classification can be made upon the basis of habit, that is, as to whether the habit of crime is inborn, acquired, or not yet formed. According to this classification then there are three main classes of criminals: (i) The instinctive or born criminal. This is a person in whom the tendency to crime is inborn, and this inborn tendency is always due to some congenital defect. The most common type of the instinctive or born criminal is the moral imbecile, a person only slightly mentally defective who cannot distinguish right from wrong. It is evident that in the instinctive or born criminal biological causes of crime predominate. This class is however relatively small among the general criminal class, and it is estimated by experts that it constitutes not more than from 10 per cent to 15 per cent of our prison population. (2) The habitual criminal. The habitual criminal is a normal person who has acquired the tendency to crime from his environment. The most marked type of the habitual criminal is the professional criminal, who is frequently a person above the average in ability and who deliberately chooses a career of crime, taking the risks of his calling. It is evident that the professional criminal class is the most dangerous class of criminals with whom society has to deal. A more common type of habitual criminal, however, is the occasional habitual criminal, a weak person who drifts into crime through temptation and who has not strength of character enough to throw off the habit. It is estimated that habitual criminals of both types mentioned constitute from 40 per cent to 50 per cent of our prison population. (3) The single offender. The single offender is a normal person who commits only a single crime through some sudden stress or temptation, but lives ever after a law-abiding life. The two types of the single offender are the criminal by passion and the accidental criminal. The criminal by passion is a moral, and oftentimes a conscientious, person who commits a crime through some sudden stress of passion, under great provocation. The accidental criminal, on the other hand, is the weak type of moral person who yields once through some sudden temptation, but who regrets it ever afterward. It is estimated that single offenders constitute from 40 per cent to 50 per cent of our prison population. Strictly speaking, they are only legal criminals, and not criminals in the sociological sense, being relatively moral and law-abiding citizens whose variation