What happens if new adaptations that have appeared in individuals are not useful for them?

According to the well-known Darwin’s law on the adaptation of species, an organ unnecessary for it never evolves in an animal, because in the process of adaptation, the organism of a creature responds to the influence of the environment, with the disappearance of an external factor, the organ will be unnecessary and if it does not disappear, it can lead to the death of the animal. An animal will never have an unnecessary organ, because this can no longer be called an adaptation – it is a mutation. As a result of mutation, an organ that is unnecessary for him may grow in the body, which may not at all affect his life.