Anyway, it isn't survival of the fittest in a civilization. So the longer a civilization lasts, the lower the abilities of the average. Like the Egyptians, whose finest work, whose greatest accomplishments, took place right after the outsiders had crossbred Cro-Magnon into homo sapiens sapiens and taught them. After that it was all downhill. Same with morals.
Immoral parents raise amoral young.
As permissiveness and promiscuity increase morals become outmoded, and then the drive for wealth and power and pleasure becomes everyone's goal. Like it is now. But decadence and self-indulgence, the results of wealth and declining morals, attract enemies. Then a newer, more dedicated and duty-driven nation moves in and takes over, through politics, economic power or even war if necessary. A snort caused the visible firmament to waver, then slowly still.