The “Use or Disuse” principle was a case suggested by Jean Baptiste Lamarck circa 1809 to explain the mechanism behind the gradual, chronic observation of evolution and variation via Lamarck’s hypothesis of the “Inheritance of Acquired Characteristics”. Lamarck suggested that individual organisms followed this principle as an explanation behind phenotypical variation within a species.
Under this principle, when an organism does or does not need something, they change their body parts to suit their needs accordingly. As an example, Lamarck compared the principle to the arms of a blacksmith. One arm, the dominant arm that the worker used to shape metal, was muscular and more developed than the non-dominant arm. Lamarck claimed that this was a case of the “Use or Disuse” principle, and that the development of these traits (ex. the developed arm) was the development of an acquired characteristic.
This is, to be fair, true in both cases; however, it should be recognized that the “Use or Disuse” principle is a human way of observing evolution. It does not explain the evolution of the long neck of giraffes, a famous example attributed to Lamarckian schools of thought. Under this line of thought and with this example, a giraffe originally possessed a short neck, but in order to reach food at a higher elevation, it stretched its neck far enough that it could reach it. This would have resulted in its development of a long neck as an acquired characteristic. Although by a modern lens, this is clearly untrue, again, this line of thought may make more sense from a human perspective towards one’s own technological development and advancement.