Sixth, class systems differ in terms of the degree of hostility which
prevails between classes. In a few instances, class warfare of the
type envisioned by Marx has prevailed, at least for a time. At the
other extreme, there has often been a virtual absence of hostility.
There is good reason for supposing that class hostility is inversely related
to opportunities for mobility, though available evidence suggests that
the relationship is far from perfect (1966, p.82).