Bodily changes acquired through use, disuse, or environmental conditions were widely thought to be inherited and to direct evolutionary change.

Lamarck made inheritance of acquired characteristics central to his evolutionary theory. Repeated use could strengthen an organ, disuse could weaken it, and those changes could then be passed to offspring. Similar ideas remained influential well beyond Lamarck; even Darwin considered mechanisms that might transmit effects acquired during life.

The important distinction that emerged later was between changes to the body and changes to the material passed to offspring. August Weismann argued that reproductive cells—the germ line—were separated from ordinary body cells. A stronger arm built through exercise changes the body, but it does not normally rewrite the inherited material in sperm or eggs.

Mendelian genetics, chromosome biology, and later population genetics then showed how evolution could proceed through inherited variants that are passed on, recombined, gained through mutation, and filtered by natural selection and chance.

Environmental conditions can still affect later generations in some cases. Maternal effects, developmental conditions, cultural inheritance, and some epigenetic changes can persist. Those effects are not the broad rule proposed by classical Lamarckian theories, in which bodily changes caused by use, injury, disuse, or need routinely become inherited evolutionary changes.