The Process of Reincarnation
Being immortal souls, we incarnate on the physical plane again and again, and during each of these incarnations, we’re given the opportunity to develop ourselves physically, mentally, and spiritually. Although this process is complex and involves various levels that include both species-oriented development and soul-oriented development, I will attempt to explain this process in the simplest terms as it specifically relates to the soul. Further explanations for some of the concepts presented are given in the endnotes.
Physical, Mental, and Spiritual Aspects of Soul Development
The physical aspect of soul development is best understood by the encoding of our DNA, where those souls who are between incarnations can choose to adopt the mix of DNA between a couple who procreate by entering the womb during the earliest embryonic stage and supply its own soul-energy to sustain the embryo so that it can continue to develop.  Every soul carries with it a morphic blueprint that reflects the level of development it has achieved over all its previous incarnations, and this includes all of those traits and propensities that make up its underlying personality. 
The mental aspect of soul development is somewhat conditional to the genetic makeup of the DNA mix that has been adopted by the newly incarnating soul, but beyond that, this soul brings with it a sort of unconscious memory of all that it has ever learned or experienced in its previous incarnations. Although this memory isn’t easily available like we understand normal human memory to be, it’s encoded into the soul’s morphic field, and can be accessed at certain times and in certain ways. When we learn or experience the same things over a number of incarnations, they become more deeply engrained on our soul’s morphic field and thus easier for us to deal with in each incarnation. Therefore, a person who happens to become a musician in various lifetimes usually learns this talent more quickly with each incarnation and becomes more proficient with less effort. In the same way, a difficult experience that’s continually encountered in various lifetimes becomes easier to deal with in each new cycle.  
This developing mental aspect provides greater awareness, understanding, intelligence, and intuition, as long as it’s given the opportunity to be exercised during each incarnation. This developing mental aspect is encoded into the morphic field of the soul and is a potential rather than a certainty, so that it requires some effort to re-establish in each incarnation, but with each incarnation it becomes easier to do so. On the other hand, a person who, in a particular incarnation, is mentally lazy and doesn’t do much to exercise the capacity for intelligence that they’re born with, never adds anything to this capacity. If this laziness continues over a number of successive incarnations, that potential becomes weaker and weaker with each incarnation, and re-establishing it begins to require more effort.
In considering all of this, it becomes obvious that intelligence plays an important part in the soul’s development. Intelligence offers greater ability to function during a lifetime, which allows for further development with greater choice in its direction. This is because intelligence involves awareness and understanding of more available choices in a given situation, which equates to greater free will.
The spiritual aspect of the soul develops through intelligence, since it takes intelligence to recognize and understand the purpose and utility of life and the journey through the many cycles of reincarnation, and spirit is nothing less than the desire to engage in this process. As intelligence grows, the spirit is encouraged further, and it becomes more desirous to continue in its journey.
Many people misunderstand what spirit is, and confuse it with the soul. Spirit might be considered either positive (good) or negative (evil), and although this is correct to some extent, these merely describe the soul’s degree of orientation towards its own development over that of other souls.  Spirit can be more accurately defined as the will and desire to experience life, and therefore to incarnate again and again. When we talk about good or evil spirits, we’re really talking about souls that have chosen to develop certain traits that either seek to create experiences that are helpful to other incarnated souls (good), or are debilitating to other souls (evil).
Spirit develops only through an effort of will and intent, where the desire to incarnate and have experiences sustains the soul as an independent entity. As we lose our spirit, we lose this desire. When we lose this desire, we do not seek to reincarnate. If we lose enough of our spirit, our soul’s energy weakens to the point that we lose the ability to incarnate at the current species-oriented level, and instead our soul’s morphic field eventually degrades to a previous (lesser) species-oriented state that requires less intelligence and the experiences of which are more compatible with that lower spiritual level. 
Spirit can become distracted from its own development and take on desires that serve the will of another. This is a dangerous situation, in that the spirit becomes entrapped through false understandings that lead it to serve that other soul as though doing so is the only way for it to develop. This is not to say that serving others in an altruistic fashion is wrong, but rather that serving another in the belief that this other is inherently greater than themselves and therefore deserving of such service leads to the eventual relinquishing of one’s own soul to that other. At some point, the energy of the serving soul is consumed by the other soul, and the serving soul no longer has any individuality. The other soul increases in soul-energy and becomes that much more vitalized.
 The act of procreation creates the stimulation of energies that attract non-incarnated souls. These energies attract by resonating with those non-incarnated souls that are best suited to the genetic blueprint of the procreating couple. This gives the non-incarnated souls some choice in incarnating, although the souls that resonate the greatest with the genetic blueprint will have priority over those that resonate less. This affects species-oriented development by assuring that the soul of a lower species will not likely adopt an incarnation into a higher species, and visa versa, although this isn’t ruled out entirely, and may explain certain defects such as mental retardation and physical deformations.
 The different genetic traits of both parents are chosen between by the newly incarnating soul according to how they will best reflect its own developmental blueprint. When the two DNA strands from each parent combine during the creation of the first embryonic cell, the energy of the newly incarnating soul naturally resonates best with the DNA combinations that reflect its own blueprint.
 Some past-life experiences that have been repeatedly experienced over various incarnations can have been so traumatizing that they stigmatize a person when they encounter a similar experience again in a new incarnation. These experiences can be very problematic and require special attention in order to learn how to get through them or avoid them.
 Although we may or may not have chosen the experiences we fall into during any particular lifetime, we become more adept at handling those experiences each time they occur.
 Negative orientation can arise when a soul recognizes its own greater development of the physical or mental aspect compared to others, or wrongly assumes so, and begins to take advantage of that for its own sake.
 Every soul progresses through stages of evolution, beginning with the simplest physical forms, and advancing through stages of development over many incarnations. Each higher stage has a more advanced morphic blueprint than the previous, and these morphic blueprints can be thought to be layered one on top of the other, so that within the morphic blueprint of any member of a species can be found the blueprints of all those previous species that it has been in its long journey through all of its incarnations.