Now these ancient thinkers of India applied this law of Karma to explain the destiny of human souls, and it was upon this law they based the theory of Transmigration. They maintained that human souls are bound by this irresistible law and cannot get out of it; their thoughts and deeds are the causes which produce results of similar nature. So their future birth does not depend upon their whimsical, free choice, but it is limited by the thoughts and deeds or misdeeds of their previous lives. In the Platonic idea we find that the souls go according to their choice. They may not take a human form if they prefer an animal form, but in the Hindu idea of transmigration we find that it is not a result of free choice, but, if our thoughts and deeds force us to take a particular form, then we are subject to the law of Karma, which governs our future birth and the evolution of our souls. Consequently the Hindu theory of Transmigration differs fundamentally from the Platonic as well as from the Egyptian idea of Transmigration. In the Platonic and Egyptian theories we see that the souls, after leaving the body, enter into another body which is waiting to receive the migrating soul, but in the Hindu theory of Transmigration the body is not waiting to receive the migrating soul, but on the contrary the soul, being subject to the laws of evolution, manufactures the gross material body according to its desires and tendencies. Just as a germ of life will develop a grosser form by cellular subdivision, by growth, and by assimilation of the environmental conditions, so the germ of the human soul will manufacture the body by obeying the laws which govern the physical plane. Parents are nothing but the channels through which the migrating souls receive their material forms. Parents do not create the souls; they have no power to create. They can only give the suitable environments necessary for manufacturing a gross physical body. The souls come with their tendencies, with their desires, and they remain as germs of life.
Now these germs of life contain vital forces, sense powers, psychic powers, and ethereal particles of matter. At the time of death the soul contracts and withdraws all its powers from the sense organs to its innermost center, and in that contracted state it leaves the body. But these powers do not leave the soul. By the law of persistence of force and conservation of energy they remain latent in that center until environmental conditions become favorable for their remanifestation. Rebirth means the manifestation of the latent powers which exist in the germ of life or in the individual soul. These germs of life are called by different names. Leibnitz called them monads and modern scientists call them bioplasms or some such name, but the Vedanta philosophers describe them as subtle bodies. These germs or subtle bodies are subject to evolution and growth; they arise from lower to higher stages of development, from the mineral through the vegetable to the animal kingdom and eventually they become human beings and then they go on progressing.