Complex sentence: The last third of his book "The Genetical Theory" concerned the applications of these ideas to humans, and presented the data available at that time and a theory that attributed the decline and fall of civilizations to the fertility of the upper classes being forced down.
Split into simpler sentences: The last third of his book "The Genetical Theory" concerned the applications of these ideas to humans, and presented the data available at that time. He presented a theory that attributed the decline and fall of civilizations to its arrival at a state where the fertility of the upper classes is forced down.