I mean, it wasn’t a horrible theory at the time, and there was even some evidence for it (because the blacksmiths sons were very likely also blacksmiths, and thus also muscular). Even Darwin couldn’t explain HOW his theory evolution was actually supposed to function, he basically just said that it did.

Unfortunately, even after we discovered genes, people were still following Lamarck, and it’s offshoot, Lysonkoism got a LOT of people killed, despite by then being verifiably false.