Species and natural selection are the two most common terms Charles Darwin uses in the book from the title—The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.

In the “Glossary of the Principle Scientific Terms Used in the Present Volume”, while Darwin defines “Organism” as “An organised being, whether plant or animal”, surprizingly, neither “species” nor “natural selection,” the key terms of the book, are defined in the Glossary. The question is why.

Defining “species” became one of Darwin’s great challenges. From the start, Darwin recognized that among naturalists of the day, the term “species” did not have a consistent definition: “No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.”

 Unlike Newton who was able to measure and test the laws of gravity, Darwin had to deal with the problem that there “is no possible test but individual opinion to determine which of them shall be considered as species and which as varieties.”

In the pursuit for a definition, Darwin suggested that as a variety begins to exceed the number of the parent species, the new variety becomes a new species: “If a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species.” In other words, species was simply a numbers game.

Darwin’s numbers game approach was never seriously taken—even by Darwin himself. After 150 years, the problem of defining species has not been resolved and is now known as the long-standing “Species Problem.”

Jody Hey of Rutgers University wrote in Trends in Ecology & Evolution (2001) – “The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word ‘species’. The innumerable attacks on the problem have turned the often-repeated question ‘what are species?’ into a philosophical conundrum.”

Massimo Pigliucci professor of Ecology and Evolution at the State University of New York at Stony Brook noted in BioEssays (2003) “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.”

The Origin of Species is loaded with plastic contradictory definitions even on the central term of the book—species. Darwin eventually concedes on the definition of species by writing – “We have seen that there is no infallible criterion by which to distinguish species and well-marked varieties.”

Today, known evidence remains compatible with the following definition of species that Darwin long endvoured to eliminate – “Generally the term [species] includes the unknown element of a distinct act of creation.”