With our human limitations, can we imagine eternity? Passing through beginningless and endless duration, we finally reach our present life and subsequent death, and then we exist once more in endless eternity. Our view of time, like that of space, depends on our vantage point. From the standpoint of eternal duration, the seemingly vast five-billion-year life of our solar system is an insignificant blip. Why, then, out of all eternity, are we at this one instant in what we call time? What do we do in the rest of eternity? Even granting that some believe we can escape the wheel of rebirth and attain the ultimate goal of nirvana, we are still left with eternity.
A classical Greek saying runs: sleep and death are brothers. It's instructive to compare death with sleep. We go to sleep at night and enter a different state of consciousness, but we awake in the morning as a continuation of the same consciousness as that of the previous day. In much the same way, reincarnation is the continuation of consciousness after death. Reincarnation is similar to the concept of a resurrected body. Where does our body resurrect? It seems logical that the place where our human consciousness reimbodies itself is this earth, the natural home to which it is attracted. As expressed by Katherine Tingley:
There is a deep instinct in the soul that this earth is its natural home and that happiness is its natural state. That is why those who have sought to describe heaven have never been able at the best to describe anything but a glorified earth. -- Mysteries of the Heart Doctrine, p. 69
The after-death state is analogous to heaven, understood to be a state of consciousness, not a locality. But instead of remaining for all eternity with our "Father" in heaven, we eventually continue (with the help of a new set of earth parents) by growing a body on earth. Each immortal spirit periodically needs to express itself in a new body. Furthermore, reincarnation provides a field (earth) for karma to be acted out. We have another chance to meet past faults and triumphs, for all levels of our being to continue their development. From a theosophic perspective, humans almost always reincarnate as humans. A continuation of the same consciousness demands a similar vehicle.
It is said by Herodotus that the ancient Egyptians believed a human soul could reincarnate as an animal. The word used by Herodotus was the Greek zoon which, like the equivalent Latin word animal, can signify "beast" -- or can signify "living being." The term is properly used for an animate being of any kind, when the emphasis is placed on that individual incarnated in a physical body. Using the original Greek sense of the word, the translation becomes: "the soul enters into another animate or living body."
Certainly there are portions of our intermediate nature or soul that are passed on to the animal kingdom, just as our physical atoms return to earth. But that is a transmigration of the more material elements of our constitution, rather than that which enlivens, inspires, and animates the body -- the reincarnating ego itself.
Because we evolve bodies to suit our needs, normally we can't go back to animal bodies. Comparing the human kingdom to the animal kingdom is like comparing the animal kingdom to the plant kingdom: a consciousness evolved to the animal state would find the vehicle of a plant inappropriate for expression of qualities -- mobility, for example -- which that soul has laboriously evolved forth from itself for eons. The vehicle must be appropriate for the being. In other words, incarnation of a human into an animal body would not be appropriate for the faculty of self-awareness we humans are bringing forth from within ourselves.
Nature is always expressing its inherent vitality. This necessitates reimbodiment. That's why we're here now. Ultimately, that is why the universe came into being. Life force will be expressed. What happens to that energy at death? Consider this analogy: the light bulb is like a physical body. The filament that glows when carrying current is like the intermediate soul. The electric current that vivifies the bulb is like the spirit -- the permanent part of us -- overshadowing every individual. When burned out, the bulb is discarded like the physical body. But the electric current still has the potential to express itself when the appropriate conditions occur again. The replacement bulb must be the right wattage and screw size. Likewise, the succeeding body must be fit to express that vitality -- so it's only natural that a human would reincarnate in another human body. The life forces within every being have the same natural, inherent urge to express themselves, to continue onward, to reincarnate.
Carrying the analogy further: When the light switch is turned off, it's like going to sleep. When the switch is turned back on, it's like awakening the next day -- in the same body as before. Furthermore, there's a wide variety of light bulbs in the world -- colored lights, incandescent, fluorescent -- which are analogous to different types and races of people. We all are of the same essence and draw the same current, yet each radiates the light of life in its own way.
The electric current that supplies every house and circuit is from a single source, from a power plant for a given area. That source is something more universal, perhaps like the unknowable, unmanifested, universal principle at the core of each being. In the theosophic view, even the "ultimate" goal and peaceful state of nirvana is not permanent -- it's an incomprehensibly long period, but in the scheme of eternity, life will eventually find a way to express itself again. From the standpoint of the core of being, nirvana is not a state of annihilation, but a state of absorption, nonbeing, oneness in which there is no individuality or separateness. Just because a power plant blows up, or the river supplying a hydroelectric plant runs dry, doesn't mean that electricity ceases to exist. Likewise, the life essence does not cease to be. It still exists in potential and will find a way to express itself again and again. It continues -- flowing ever onward to the sea, water transforming to subtle vapor, to rain down another time on the headwaters, expressing itself again and again.