As population increases, animal souls are promoted to human before they are spiritually advanced enough in terms of impulse control, etc.: this shouldn't have happened, but karmic progression is automatic, after all--it's a force of nature. But the 'animalization' of humankind bodes ill for civilization.
No doubt the same goes for elves, dwarves, and so on.
"The Cetemi have a most curious custom that in their celebrations must all men don the garb of women and women... the garb of men, to the aim that none shall... know another." -An Account of Barbarian Lands, Lord Shakthur