What two key insights allowed darwin to come up with the therory of natural selection?

The key insights were that any species develops varieties of which some are more adapted to the environment than others. Along with this was the changes in the environment or the physical world by natural geological processes such as the creation of savannas from forested areas or also from the abilities and type of predators that would favor say man's upright gait in the former instance and a fleet footed coyote on the other hand to escape a predator. 

Bc they get tired from playing with other kittens