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. When the lunar chain came to its end the humanity upon it stood at various levels. Not Adeptship, but what is now for us the fourth step on the Path, was the goal appointed for that chain. Those who had attained it (commonly called in Theosophical literature the Lords of the Moon) had, as is usual, seven choices before them as to the way in which they would serve. Only one of those choices brought them, or rather a few of them, over into this Earth-chain to act as guides and teachers to the earlier races. A considerable proportion—a vast proportion, indeed—of the Moon-men had not attained that level, and consequently had to reappear in this Earth-chain as humanity. Besides this, a great mass of the animal kingdom of the Moon-chain was surging up to the level of the individualization, and some of its members had already reached it, while many others had not. These latter needed further animal incarnations upon the Earth-chain, and for the moment may be put aside.
There were many classes even among the humanity, and the manner in which these distributed themselves over the Earth-chain needs some explanation. It is the general rule that those who have attained the highest possible in any chain on any globe, in any root-race, are not born into the beginning of the next chain, globe or race, respectively. The earlier stages are always for the backward entities, and only when they have already passed through a good deal of evolution and are beginning to approach the level of those others who had done better, do the latter descend into incarnation and join them once more. That is to say, almost the earlier half of any period of evolution, whether it be a race, a globe or a chain, seems to be devoted to bringing the backward people up to nearly the level of those who have got on better; then these latter also (who, in the meantime, have been resting in great enjoyment in the mental world) descend into incarnation along with the others, and they press on together until the end of the period.