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. The second three establish that this struggle will affect fitness, which is a quantitiative measure of realized reproductive potential.