He proposed the theory of inheritance of acquired characteristics, also known as Lamarckism. This theory stated that organisms could pass on traits they acquired during their lifetime to their offspring. He famously used the example of a giraffe stretching its neck to reach higher leaves, with the lengthened neck being passed on to its offspring.

While this theory is now largely discredited, it was an important step in the development of evolutionary theory and helped set the stage for Darwin's ideas.