The very ancient belief that certain people were empowered with the magical ability to change themselves and others at will into other life forms such as other human beings, animals, birds and insects.
Others, such as those believing in ancient Druidism, considered metamorphosis a part of reincarnation. Each reincarnation was considered to be a learning experience for the spirit to learn new knowledge. This was accompanied by the belief that death is not permanent, but only the exit from one life to enter the next.
The practice of shamanism contain metamorphic characteristics. The practitioners exist in two worlds: the awake or conscious world, and the dream or unconscious world. To enter into the latter world the practitioner enters a trance.
(Some modern theorists think the Celts, and especially the Druids practiced some form of shamanism. It is fairly well agreed that they practiced a form of sexual magic. This may have put them into an unconscious state. Others claim this Celtic shamanism may have spurred the development of European sorcery and witchcraft.)
There are many reasons for the practice of shamanism, but one of the most common is for the healing of the sick. A sick person comes to a shaman often to discover the cause of his illness. To discover this cause the shaman enters a shamanistic trance in which he can see and do things which he is not able to do in the world of reality.
Often, it is said, the shaman enters the lowerworld beneath the earth by slipping through a hole within the earth. Once in this subterranean world his shamanistic powers quicken. Also, it is here that he meets what is known as his guardian spirit and spirit helpers who guide him to find the cause and cure of his patient's illness. In the shamanistic world he can shape-shift into other forms, even into the spirits themselves.
Once his mission has been accomplished, the shaman returns to the real world. Some shamans also claim to have ridden up into the sky in spirit boats or astride the spirits of sanctified horses
Although shamanism, a very ancient practice, is a very good example of metamorphosis, it is still practiced in tribal cultures, and among various occult groups throughout the world.
And, metamorphosis is still present in the minds of modern men. Many "Star Trek" fans may remember the episode where metamorphosis occurred, Captain Kirk had a tough fight on his hands.