It is a probable hypothesis that what the world is to organisms in general, each organism is to the molecules of which it is composed. Multitudes of these having diverse tendencies, are competing with one another for opportunity to exist and multiply; and the organism, as a whole, is as much the product of the molecules which are victorious as the Fauna, or Flora, of a country is the product of the victorious organic beings in it.
Darwin wrote back on October 14 1869:
I am very glad that you have been bold enough to give your idea about Natural Selection among the molecules, though I cannot quite follow you
This early exchange makes T. H. Huxley an early pioneer of universal Darwinism.
The details were not all fully understood at the time - but we now know that molecules are the product of an extended evolutionary history - which includes splitting and merging events as well as iterated selection. Splitting was always a fundamental feature of Darwinism - but it really wasn't until the symbiology revolution of the 1960s-1980s that the significance of mergers to evolution was understood. Iterated selection is a central characteristic of Darwinism. Molecules exhibit this extensively - with every molecule the survivior of a long series of potentially lethal selection events. The stability of observed molecules can be usefully understood as a form of Darwinian adaptation. Unstable molecules are possible an theory - and are sometimes observed fleetingly - but the law of survival of the stable means that most of them are short-lived.