The Weismann barrier states that hereditary information normally passes from germ cells to body cells, not from acquired body changes back into germ cells.
German biologist August Weismann developed the idea in the late nineteenth century while arguing against the inheritance of characteristics acquired during an organism’s lifetime. A classic example would be a muscle gained through exercise not automatically being encoded into the offspring’s DNA.
The principle helped separate inherited variation from changes caused by an individual’s environment or behavior. It became an important foundation for modern evolutionary biology and the distinction between germ-line and somatic cells.
The barrier is not a claim that environments never affect descendants. Environmental conditions can influence offspring through mechanisms such as altered development or some forms of epigenetic inheritance. Rather, the principle describes the usual direction of genetic information flow and does not support Lamarck’s broad inheritance model.