The central idea of Lombroso's work came to him as he autopsied the body of a notorious Italian alleged criminal named Giuseppe Villella.
The term Lombroso used to describe the appearance of organisms resembling ancestral (prehuman) forms of life is atavism.
Lombroso believed that atavism could be identified by a number of measurable physical stigmata, which included protruding jaw, drooping eyes, large ears, twisted and flattish nose, long arms relative to the lower limbs, sloping shoulders, and a coccyx that resembled "the stump of a tail."
If humankind was just at one end of the continuum of animal life, it made sense to many people that criminals — who acted "beastly" and who lacked reasoned conscience — were biologically inferior beings.
Given the assumptions of biological positivism, the only reasonable rationale for punishing offenders is to incapacitate them for as long as possible so that they no longer posed a threat to the peace and security of society.
This repudiation of free will (and, therefore, of moral responsibility) and fitting the punishment to the offender would eventually lead to sentencing aimed at the humane and liberal goals of treatment and rehabilitation.
Professional criminals were psychologically normal individuals who utilize the hedonistic calculus before committing their crimes, and thus require "elimination," either by life imprisonment or transportation to a penal colony overseas.