In the subhuman creation, reincarnation of evolved consciousness takes place through the subtle matrix, from which at a later stage a fully formed subtle body develops. Plants and animals do not have a fully developed subtle body, but the rudimentary subtle matrix gradually takes shape according to the stage of development achieved. This subtle matrix is the vehicle for the transmigration of subhuman consciousness from one species into others. It is also the medium in which the subhuman creatures live in the astral before taking a new gross form.
As a general rule the life-stream advances through opposites. For example, those men who have once appeared with mental brilliance in the gross world are sometimes devoid of that mental brilliance in a subsequent incarnation. Of course, the capacities of mental brilliance which they have once acquired are retained in the mental body and are in no way destroyed. But the intrinsic logic of the spiritual advancement of the soul may entail for it a period in which these capacities of mental brilliance are not accessible to it. The opposites of experience are equally necessary for the full unfoldment of spiritual understanding.
Once a soul acquires the human form, the general rule is an onward march through human incarnations only. Retrograde incarnation is extremely rare in the advancing self-fulfillment of the life-stream. It sometimes results from flagrant misuse of occult powers. Retrograde incarnation is not
a general rule but an extremely rare exception. Continuation of the human form without revision to any subhuman form is the normal occurrence. Even for gross violation of moral laws a corrective is provided by the same soul having to go through the opposite of what he has done or experienced. For example, a person who murders another for lust might himself be murdered for lust, either in the same incarnation or in the next, and thus realize the nefariousness of his heinous crime.
However, retrograde incarnation can take place in those exceptional circumstances where occult powers are used for the perpetration of heinous crimes. A soul which is invested with occult powers (due to his station on a higher plane) is expected to have them under full control. But if he grossly misuses them due to being a victim of his lower desires or ego-projects, he invites upon himself the drastic penalty of having to revert back to the subhuman form and recommence his evolution from that point. This sort of thing can happen on the fourth plane unless there is a timely intervention by some higher beings. But retrogression of this type is very unusual. The forward march of the advancing life stream brings with it an increasing ascent towards the peak of realized Truth, and an increasingly greater unfoldment of inner divinity.
-Beams, 24