Perhaps the most powerful cause of the breakdown of the closed society was the development of sea-communications and commerce. Close contact with other tribes is liable to undermine the feeling of necessity with which tribal institutions are viewed; and trade, commercial initiative, appears to be one of the few forms in which individual initiative and independence can assert itself, even in a society in which tribalism still prevails. These two, seafaring and commerce, became the main characteristics of Athenian imperialism, as it developed in the fifth century B.C. And indeed they were recognized as the most dangerous developments by the oligarchs, the members of the privileged, or of the formerly privileged, classes of Athens.