In the vegetable kingdom, for example, the life-force might commence its career by occupying grasses or mosses and end it by ensouling magnificent forest trees.

In the animal kingdom it might commence with mosquitoes or with animalculae, and might end with the finest specimens of the mammalia.
The whole process is one of steady evolution from lower forms to higher, from the simpler to the more complex.

But what is evolving is not primarily the form, but the life within it.

The forms also evolve and grow better as time passes; but this is in order that they may be appropriate vehicles for more and more advanced waves of life.

When the life has reached the highest level possible in the animal kingdom, it may then pass on into the human kingdom, under conditions which will presently be explained.
The outpouring leaves one kingdom and passes to another, so that if we had to deal with only one wave of this outpouring we could have in existence only one kingdom at a time.