Some souls are so tiny and poorly constructed that they dissipate after the incarnation, what we term 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.