Formulation of Evolutionary Laws

The first law stated that use or disuse would cause body structures to grow or shrink over the generations: more frequent and continuous use of an organ strengthens, develops and enlarges it, while permanent disuse weakens and deteriorates it until it finally disappears. The second law held that acquisitions or losses wrought by nature through the environment and through predominant use or permanent disuse of any organ are preserved by reproduction to new individuals, provided the acquired modifications are common to both sexes or at least to the individuals producing the young. The last clause introduces what is now called soft inheritance, the inheritance of acquired characteristics, or simply "Lamarckism", though it forms only part of Lamarck's thinking.