The substance that makes up the spirit is called is not something that the human eye can perceive. The soul has substance, and grows in bulk early in its development as many incarnations, many lives, are experienced. This bulk cannot be gauged by humans, who look upon one another as having equal souls, when nothing of the kind is the reality. Some souls are so tiny and poorly constructed that they dissipate after the incarnation, what is termed aborted entities. This type of soul may be present in many animals and even some plant forms, as the stuff of souls is dispersed throughout the Universe and incarnations happen naturally. Where the human eye cannot gauge the mass of another soul, the spirits gauge this very well. You know instinctively that another is old and wise. Spiritual growth occurs most rapidly early on via incarnations, but later this growth proceeds based on experiences outside of incarnations, and incarnations are not needed for this purpose. Incarnations continue to be used as a learning experience, however, from time to time.
The soul not only has memory, it does not forget. It is not by accident that intelligent life forms have nerves, brains, and memory in the form of chemical impressions. It is not by accident that this same structure is found for intelligent thought on all density levels. It is intrinsic to the way the Universe is structured, and thus souls likewise have such structures. The stuff of souls is simply another density level, one that touches on all the others. It is finer and more durable, at the same time. It could be called the base of matter, in that regard, as it permeates all levels where matter can reside. Intelligent life forms come in many shapes and sizes, but all have brains and nerves. Thus a soul can incarnate in a hominoid at one point in its development, and into quite another life form during another incarnation, without any adaptation required.
The soul does not require food or nourishment as physical life forms do. The human animal cannot retain its shape without fuel, without maintaining a certain temperature and replacing damaged or consumed biochemicals. It must eat to survive. Souls do not require a temperature maintenance, and by their nature maintain their own chemistry, the only outside reach required is to acquire more of the Chromatic substance of souls, present everywhere in the Universe, when growth in bulk is occurring.