35. In the interval between their material existences, spirits live in an erratic state, and compose the ambient spiritual population of the globe. By deaths and births those two populations are mingled. There is then daily emigration from the material to spiritual world, and immigrations from the spiritual into the material world. This is the normal state of existence.
36. At certain epochs regulated by divine wisdom, these emigrations and immigrations take place en masse in consequence of great revolutions, causing great numbers of human beings to change worlds, which are soon replaced by equivalent incarnations. It is then necessary to consider destructive scourges and cataclysms as collective arrival and departures - providential means of renewing the material population of the globe by replenishing it by the introduction of new and purer spiritual elements. If in these catastrophes there is a great destruction of bodies, they are only torn vestments; but no spirit perishes. They only change place. Instead of departing alone, they go in numbers, which is the whole difference from the ordinary. By one cause or another they must inevitably depart sooner or later.
The rapid and almost instantaneous renovations which take place in the spiritual element of the population, in consequence of destructive scourges, hasten social progress. Without emigrations and immigrations, which from time to time give a violent impulsion, it would march very slowly.
It is remarkable that all great calamities which decimate populations are always followed by an era of marked spiritual, intellectual and physical progress in the social state of the nations in which they occur. Their object is to produce a great change in the spiritual, which is the normal and active, population of the globe.
37. The transfusion, which takes place between the incarnated and spiritual population of the same globe, operates in the same way between different worlds, either individually in normal conditions or by masses in special circumstances. There are then collective emigrations and immigrations from one world to another. From there it results in the introduction into the population of the globe of entirely new elements, new races of spirits coming to mingle with existing races, constituting new races of men. Now, as spirits never lose anything they have once acquired, they carry with them intelligence and intuition of the knowledge they possess. They impress, consequently, their character on the corporeal race they came to animate. There is no need of new bodies created for especial use. Since the corporeal species exist, they find them ready to receive them; they are simply new inhabitants. In arriving upon the Earth, they are at first a part of its spiritual population; then incarnate themselves like others.