What kind of trait is that, exactly?No, Darwinian evolution isn't based on acquired traits, that's Lamarckism.Imagine a savanna populated by primitive giraffes, with shorter necks than the modern ones. Then, some catastrophe happens and the giraffes can only eat from tall trees, that not every giraffe can reach.
According to darwinism, shorter necked giraffes die, longer necked giraffes survive and repopulate the dead giraffes, now almost every giraffe can reach those trees.
According to lamarckism, the shorter necked giraffe's neck will grow until they reach the leaves, and their offsprings will inhert longer necks. We consider darwinism correct and lamarckism wrong nowadays.
The living creatures with the required traits survive, the others die. The survivors procreate, and now only there's predominantly this kind of creature. The did not acquire that trait along their life, at least if the trait is an organ