  A small fraction of double radio sources show a peculiar and striking hybrid
morphology; they have a distinctly FR I structure on one side of the nucleus,
and a FR II structure on the other. We argue that the mere existence of these
HYMORS is quite incompatible with the theoretical explanations for the
Fanaroff-Riley dichotomy that are based upon the nature of the jet plasma, or
those invoking an intrinsic property of the central engine. Rather, these
HYMORS strongly support models that explain the difference between FR I and FR
II sources in terms of asymmetry of interaction of the jets with the external
environments. We further show that a model for radio source dynamics we had
earlier proposed can neatly reproduce the observed dependence of the radio
power dividing the two FR classes on the optical luminosity of the host galaxy,
as found by Owen & White and Ledlow & Owen
