In the time between their physical existences, the spirits are in a state of impermanence and form the spirit population of the environment of the globe. Through death and birth, the two populations intermingle incessantly. Thus, there are daily emigrations from the physical world to the spiritual world and immigrations from the spiritual world to the physical world: this is the normal state of affairs.

In certain epochs, regulated by divine wisdom, these emigrations and immigrations occur in greater or lesser numbers as a consequence of the great upheavals which cause their simultaneous departure in immense numbers, which are immediately replaced by a corresponding number of incarnations. Destructive scourges and catastrophes must therefore be regarded as occasions of collective influxes and outflows, as providential means of renewing the physical population of the globe, which is strengthened by the introduction of new and purified spiritual elements. Though in these catastrophes a large number of bodies are destroyed, they are but torn garments, for no spirit perishes; they hardly change their environment. The only difference is that they do not depart individually, but in great numbers, for for one reason or another they must depart sooner or later.

The rapid, almost instantaneous renewals which take place in the spiritual element of the population as a result of the destructive scourges, accelerate social progress; were it not for the emigrations and immigrations which come from time to time to give it a violent impulse, this progress would only be realised with extreme slowness.

It should be noted that the great catastrophes which decimate peoples are always followed by an epoch of progress in the physical, intellectual or moral order, and consequently in the social condition of the nations in which they occur. This is because they are intended to bring about a change in the spiritual population, which is the normal and active population of the globe.

The transfusion which takes place between the incarnated and disincarnated populations of the same globe also takes place between worlds, either individually under normal conditions or on a massive scale under special circumstances. There are thus collective emigrations and immigrations from one world to another, leading to the introduction of entirely new elements into the population of a globe. New races of spirits mingling with existing ones form new human races. As spirits never lose what they have conquered, they carry with them the intelligence and intuition of the knowledge they possess, thus moulding their character into the physical race they are to animate. For this, they do not need new bodies created exclusively for them. The corporeal species exists, so they will always find bodies ready to receive them. Therefore, they are hardly new inhabitants. When they arrive on Earth, they first integrate the spiritual population and then incarnate as others.