Every race and sub-race offers a different but particular leanring opportunity as characterised by the race or sub-race. Every ego therefore is born (rather, must be born) in every race and sub-race, at least a number of times before it fully learns what the sub-race or race has to offer. This does not happen in a natural sequence, but according to karma. It is for this reason someone may be born in a previous race or sub-race in his next life.
This non-sequential birth happens in only close-by races or sub-races. That is, one who is born into the Fifth race may take a birth in the seventh sub-race of the fourth race, but rarely or never in the Third race or Second race.
There is another aspect. The initial races are populated by primitive egos. In later races, the primitive egos drop out of evolution and await in the inter-Globe realms for the next Goble to resume their evolution, while the advanced egos that awaited for the right conditions now come in “truck loads” to resume thier evolution. These advanced egos, obviously, have learn the lessons about the initial races in a previous Globe.