Darwin assumes as obvious the dependence of fitness on character state, and so includes it as an unspoken correlate of variability. And he leaves finite resources as an unspoken but self-evident fact of life in the real world. But any way it's worded, Natural selection is what happens whenever these basic conditions obtain. It is a label given to the necessary outcome of life under these conditions. Note that in this last version two terms carry a lot of freight.
Heritable variation encompasses reproduction, genetics, development and mutation.
Fitness encompasses all of the struggle for existence. Potential for population growth. Finite resources which constrain indefinite population growth. A source for variation in heritable traits. The first three of these yield a struggle for existence in which some reproductive potential is not achieved.