First the word itself: what does it mean? Trans means across; to migrate means to travel into a new country. What does it apply to? It applies to the tiny, tiny lives which go to make up not only man's body, but all his psycho-mental and emotional nature. What are these tiny lives? They are called by science Atoms, but Theosophy goes one step farther and calls them Life-Atoms. These Life-Atoms reside in the heart of and build the physical atom. They are so numerous that they make up everything that is in the Universe, visible and invisible. They help build man's body, his emotional fabric; his thoughts work through them, and there are even life-atoms active in his spiritual life. So much in brief as to what man and his life-atoms are.
The thing to remember is that a life-atom is the 'Soul,' the active principle of the physical atom. Without this 'Soul' or 'Heart-life' an atom would be a mere empty shell, incapable of any action whatsoever.
How do these life-atoms operate? During a man's life, they are limited in their action because the man dominates them. By his stronger will, intellect, and spiritual powers, he controls these tiny lives. They absorb their life from him, just as we live in the Universe and absorb life from it. If we live in harmony, we generate constructive entities; if we lead misshapen lives, we give birth to destructive forces. During our life they must travel as we travel. It might be likened to many people on a railroad train under the dominance of the engineer; we can do nothing but stay on it and do as he says till we come to the end of the journey. If the engineer and the train-crew are friendly, if the train runs smoothly, if the passengers are congenial, we benefit. If the contrary is the case, we leave the train ill and unhappy. Thus it is with the life-atoms we control. It is really a Brotherhood; we help or hinder them and they help or hinder us.
How do they transmigrate? When a man dies his body disintegrates. The atoms and life-atoms which compose it are freed, and go wherever they find attraction. They go to the place or to the thing they like best or are most nearly akin to. Some go to build the soil, others help build the flowers, the trees; some help build the bodies of animals and other lower creatures. Some go to help build the bodies of other human beings. Those which absorbed the good things from us go to help the beautiful things in Nature. Those which absorbed weaknesses from us, from our evil thoughts and actions, help to build the ugly things in Nature. Those which acted through our emotions, help build the emotions of other people, or of animals.
The most subtil reaction comes from the mental fabric of the man who was. If his thoughts were of the hateful, jealous, murderous type, the mental life-atoms help build the thought-fabric of criminals, dope-fiends, drunkards and weaklings. If his thoughts were centered around acts of beauty, sympathy, understanding, compassion, if in general his thoughts were of a harmonious, kindly, gentle type, these life-atoms help build the mental fabric of the gentler and more advanced beasts, and the higher intellectual and spiritual type of human beings. The whole teaching of Transmigration hinges on the activities of the life-atoms — it is they which transmigrate — not the Soul of Man. The Soul of Man cannot possibly ever retrograde into the beast-kingdom. The saying is "Once a Man, always a Man." The goal of man is Divinity. The Soul of Man reincarnates: the Life-Atoms transmigrate. Involved in this teaching is the basis and foundation of one of the aspects of Moral Ethics and Universal Brotherhood.
"We believe in no creation, but in the periodical and consecutive appearances of the universe from the subjective on to the objective plane of being, at regular intervals of time, covering periods of immense duration." — H. P. Blavatsky
The Theosophical ForumTHEOSOPHICAL UNIVERSITY PRESS ONLINE