The unity of life comes from the fact that all living things come from a common origin-- this is the most sensible way to explain things like the universality of the genetic code. The diversity of life comes from the generation of variation over time--things are related, but not exactly the same, allowing their descendants to adapt to changing conditions.
Work Step by Step
Review past chapters, skimming them for the mechanism such as mutation and crossing over which generate the variation/diversity, and then look, for the shared rules and principles which maintain the unity of life. Descent shows a connection while modification shows diversity.