Transmigration and Reincarnation

I've heard that the soul resides in the heart of each living entity. I am a cardiac surgeon. During a cardiac transplant, is the spiritual spark from one body transferred into another?

Hare Krishna! Thanks for your question. You must have many realizations in your profession!

In Srila Prabhupada's purport to Bhagavad-gita 2.17, he says:

"The influence of the atomic soul can be spread all over a particular body. According to the Mundaka Upanishad, this atomic soul is situated in the heart of every living entity, and because the measurement of the atomic soul is beyond the power of appreciation of the material scientists, some of them assert foolishly that there is no soul. The individual atomic soul is definitely there in the heart along with the Supersoul, and thus all the energies of bodily movement are emanating from this part of the body. The corpuscles, which carry the oxygen from the lungs, gather energy from the soul. When the soul passes away from this position, the activity of the blood, generating fusion, ceases. Medical science accepts the importance of the red corpuscles, but it cannot ascertain that the source of the energy is the soul. Medical science, however, does admit that the heart is the seat of all energies of the body."

The soul resides in the heart but is not bound to the heart. If you live in an apartment and your building must be torn down, you'll find another place to live. It's not that you're bound to remain there no matter what. The soul resides in the heart of the living being. If the body dies and its heart is transplanted into another body, the soul is also "transplanted," but not necessarily within the same heart. Guna (the qualities and character exhibited by that soul) and karma will determine the soul's next destination.

Another consideration is that the fact that the soul resides in the "heart" of a living entity. The "heart" in that statement is also a synonym for "core" of that person, rather than meaning the literal heart, which is a muscular organ in all vertebrates responsible for pumping blood through the blood vessels by repeated, rhythmic contractions.

When we say, "get to the heart of the matter," we mean get to the essence. So Krishna is in the center, the core, the heart of the living entity, and the essence of the body is the jiva. It is that core being around which the body is fitted and from which the energy of the entire body is emanating.

I hope this is helpful!
Your servant,
Laxmimoni dasi

If our actions determine our next body, how do souls in animal bodies, performing totally animalistic activities, evolve to higher species?

Animals don't generate Karma. They simply "evolve" through various species in a predetermined order until they reach the human form of life.

Although they don't generate good and/or bad karma they do benefit, however, from eating prasadam (sanctified food), touching the water from the lotus feet of the Lord or His devotees and hearing the Holy Name. So, by putting other living beings in contact with these very potent items of bhakti even lower species are relieved of karmic reaction and sometimes take a human birth immediately, skipping the general progression of their evolution to the human form of life.

I hope this is helpful.
Your servant,
Laxmimoni dasi

How is it that we've had previous births?

Our Answer:

The soul is what takes birth again and again.

The Bhagavad-gita's Second Chapter explains that the soul takes on new bodies as one puts on new clothes and then removes them when they become old and useless. The body is temporary. The soul is eternal. The soul takes up residence in a body, due to past karma and according to the will of the Lord. And, although the soul remains "bound" to the body for that lifetime, when the body ages and/or becomes useless, then the soul takes on another body, unless that soul has achieved perfect harmony with the Supreme Lord, at which time the soul returns to the spiritual world and doesn't have to take birth again.

Is there a scientific explanation for "changing bodies"?

Our Answer:
Anyone can prove to themselves, using scientific methods, that they perpetually remain the same conscious entity while the body perpetually changes. It's simply a matter of hypothesis and observation. Depending on what kind of science you accept as valid, the Bhagavad-gita gives a thoroughly scientific explanation of transmigration of consciousness through different bodies, especially in the eighth chapter. You may also want to examine Dr. Ian Stevenson's three thousand case studies of children remembering past lives.

A conscious being, atma, may conceivably be transferred to any part of the material world in the next life. One's destination after death is based on the decision of God and His representatives. Our karma and our desire is factored in.

The soul leaves the body at death and does not remain in the body.