Why does the separation between germ cells and body cells challenge Lamarckism in animals?

The separation between germ cells and body cells, a concept established by August Weismann, fundamentally challenges Lamarckism because it dictates that only germ cells (sperm and eggs) produce the next generation.1

In animals, the body—or soma—and the germ plasm are distinct. Lamarckism relies on the premise that physical modifications acquired by an organism during its lifetime through activity or environmental influence can be transmitted to its offspring. However, because the germ cells are separate from the somatic cells, any changes occurring in the body do not automatically transfer to the germ plasm.12

For an acquired characteristic to be inherited under the Lamarckian model, a modification would need to be transmitted from the soma to the germ plasm and subsequently to the offspring's soma. Modern biological evidence confirms that such transmission of purely bodily modifications does not occur in higher animals, as the germ material already contains the inherited units for development, independent of the parent's somatic experiences.134