Or should we take his biological emphasis this way: a culture's life cycle has a growth and a decline phase, and the slaves are a necessary part of that cycle, and so to be evaluated simply as one phase in the cycle of life?
(the slaves; are; a necessary part of that cycle) (a culture's life cycle; has; a growth and a decline phase)
the slaves a culture's life cycle
are has
a necessary part of that cycle
a growth and a decline phase