How did Lamarckism explain the effects of using an organ often?

Lamarckism, a theory proposed by Jean-Baptiste Lamarck in 1809, posited that organisms adapt to their environment by developing new habits, which in turn influence the use of their organs.123

According to this theory, when an animal frequently uses a specific organ to meet environmental needs, that organ undergoes physical changes, such as an increase in size or improved functionality. Conversely, the disuse of an organ leads to its decline or weakening. Lamarck argued that these material alterations—referred to as acquired characteristics—could be transmitted to the organism's offspring, leading to gradual evolutionary change over generations.231

It is important to note that this theory has been scientifically discredited. Modern genetics has shown that changes occurring in an individual's somatic (body) cells during their lifetime are not passed on to their progeny through inheritance. While epigenetics now provides a more nuanced understanding of how certain environmental factors can influence gene expression, the classical Lamarckian mechanism of inheriting physical traits acquired through use or disuse is not supported by biological evidence.1324