How does evolution explain the diversity of life, while God explains that existence of life in the first place?

In On the Origin of Species, Charles Darwin argues that God functions as the primary cause of the universe, while evolution functions as the secondary cause of the universe.

What does Darwin mean by this distinction?

How does evolution explain the diversity of life, while God explains that existence of life in the first place?

Do you think Darwin would have agreed with the Big Bang Theory as the primary cause of the universe, or do you think he still would have argued for a creator God – why or why not?