This is the 3056th most frequent French word.
mécanisme
A system of parts working together in a machine or process.
In this sentence, 'mécanisme' refers to the physical mechanical system within the clock that allows it to function.
Le mécanisme de cette horloge ancienne est très complexe.
The mechanism of this old clock is very complex.
Here, 'mécanisme' is applied metaphorically to describe the underlying process or functioning ideas behind natural selection.
Il a étudié le mécanisme de la sélection naturelle.
He studied the mechanism of natural selection.
In this context, 'mécanisme' refers to a conceptual framework or set of interactions that dictate how a particular system or problem operates.