What were Darwin's two fundamental insights about the process of evolution?

What were Darwin's two fundamental insights about the process of evolution?
 
  a. The environment selects on variation in the traits of individual organisms; and organisms are in a constant struggle for existence.
  b. The environment selects on variation in the traits of individual organisms; and all species have descended from one or a few common ancestors.
  c. All species have descended from one or a few common ancestors; and over time populations, not individuals, evolve.
  d. Over time populations, not individuals, evolve; and evolutionary change is connected to envi-ronmental fit.