Darwin did not define the term “species” in the text or in the“GLOSSARY OF THE PRINCIPAL SCIENTIFIC TERMS” in The Origin of Species.
Darwin concluded that the term “species” was not definable:
It is all-important to remember that naturalists have no golden rule by which to distinguish species.
[There] is no possible test but individual opinion to determine which of them shall be considered as species.
Defining the term species is now known as “The Species Problem“.
First, the species problem is not primarily an empirical one, but it is rather fraught with philosophical questions that require – but cannot be settled by – empirical evidence.