Evidence

AuthorD. P. Lyle
Pages19-44
Evidence 19
CHAPTER 3
EVIDENCE
Wherever he steps, whatever he touches, whatever he leaves, even
unconsciously, will serve as a silent witness against h im. Not only his
fingerprints or his footpri nts, but his hair, the fibers from his clothes,
the glass he breaks, the tool ma rk he leaves, the paint he scratches,
the blood or semen he deposits or collects. All of these and more,
bear mute witness again st him. This is evidence that does not forget.
It is not confused by the excitement of the moment. It is not absent
because human witnesses a re. It is factual evidence. Physical evidence
cannot be wrong, it cannot per jure itself, it cannot be wholly absent.
Only human failure t o find it, study and understand it, can
diminish its value.
Paul L. Kirk, Crime Investigation: Physical Evidence and the
Police Laborat ory, Interscience Publishers, Inc., New York, 1953
Locard’s Exchange Principle
Professor Edmond Locard (1877–1966) was a student of both medicine and
law in Lyon, France, and served as an assist ant to Alexandre Lac assagne,
a well-known and respected crimina l investigator and professor. Profes-
sor Locard developed the twelve-point fingerprint identification system,
20 Evidence
wrote the massive seven-volume Traité de Cr iminalist ique, and in 1910,
while working at the Lyon Police Department, developed the world’s first
crime lab. But despite all these accomplishments, he is best known for his
Exchange Principle.
Locard’s Exchange Principle is the basis, the heart and soul, of forensic
science, and understanding this principle is critical to gra sping the true
workings of forensic investigation. As so elegantly stated above by Paul L.
Kirk, the basic premise is that whenever a person comes into contact with
another person, object, or place, an exchange of materia ls takes place. At
any crime scene, blood and other bodily fluids, f ibers, hair, fingerprints,
and shoe prints are deposited or picked up and carried away by anyone
present. If you own a pet, the hairs you find on your clothing are perfect
examples of this exchange.
This linkage is the basic function of forensic science. Linked evidence
proves that a person has come into contact with another person, place, or
object. The analysis of finger prints, blood, DNA, fibers, dirt, pla nt materi-
als , paint , glass , shoe an d tire i mpressio ns, and i ndeed ev ery tes t perfor med
by the crime lab is to create an assoc iation between the perpetrator and the
crime scene or other elements of the crime (victim, weapon, etc.). In some
cases, the simple fact that the suspect was at the scene implies guilt. A fi n-
gerprint on a broken window or pried-open filing cabinet, semen obtained
from a rape victim, or blood shed at the scene by the perpetrator might link
him to a location where he had no “innocent” reason to be.
Evidence linkage can a lso prove that two objects or substances share a
common source. If a paint chip found in the clothing of a hit-and-run vic-
tim is matched to a particula r car, this match shows that the car was the
source of the paint. If blood, semen, or some other bodily fluid found at a
crime scene matches the DNA profile of a suspect, it proves these materials
shared a common source—the suspec t.
Does this make either suspect g uilty? Not necessarily. That determina-
tion will be made in a court of law. The linkage of the evidence simply puts
the suspect at the scene. It will be up to the police and prosecutors to prove
that this linkage is proof of g uilt—or conversely, for the suspect and his
defense attorney to offer an innocent reason for the evidence to be found
where it was.

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