According to Darwin, existing life forms resulted from the modification of pre-existing life forms through the interaction of two factors. What are those two interacting factors?

Variation traits and natural selection.

Explanation:

Darwin stated that all species of organisms arise and develop through natural selection and inheritance of variations

In natural selection, survival was for the fittest. In order for an organism to survive. It had to develop new traits to adapt to its environment.

The variation increases its ability to compete, survive and reproduce. Through reproduction the desires trait is passed down making the organism to be considered fit.