Consequently the human race as a whole is very little more than half-way through its evolution, and those few souls who are already nearing Adeptship, which is the end and crown of this evolution, are very far in advance of their fellows.
How do they come to be so far in advance?
Partly and in some cases because they have worked harder, but usually because they are older egos--because they were individualized out of the animal kingdom at an earlier date, and so have had more time for the human part of their evolution.
Any given wave of life sent forth from the Deity usually spends a chain-period in each of the great kingdoms of Nature.

That which in our first chain was ensouling the first elemental kingdom must have ensouled the second of those kingdoms in the second chain, in the third of them in the Moon-chain, and is now in the mineral kingdom in the fourth chain.

In the future fifth chain it will ensoul the vegetable kingdom, in the sixth the animal, and in the seventh it will attain humanity.
From this it follows that we ourselves represented the mineral kingdom on the first chain, the vegetable on the second, and the animal on the lunar chain.

There some of us attained our individualization, and so we were enabled to enter this Earth-chain as men.

Others who were a little more backward did not succeed in attaining it, and so had to be born into this chain as animals for a while before they could reach humanity.
Not all of mankind, however, entered this chain together.