The concept of species is a very old concept that suffers from not being a natural category. There exists no single definition that would categorize living beings into groups and that would fit our intuitions of what a species should be.

Many of such problems in definitions are revealed in the field of evolutionary biology. For more information about the difficulties behind the definition of the concept of species, have a look at


  • Did Charles Darwin comment on this problem?
  • If yes, did Charles Darwin comment on the reason why he chose to keep using the term "species" instead of simply "lineage"?
  • 1
    $\begingroup$ On the contrary, species seems a fairly natural and obvious categorization. The problem is with people who insist on rather too narrow a definition, e.g. deciding that Neandertals should be a different species from modern humans, when the evidence shows otherwise. $\endgroup$
    – jamesqf
    Apr 11, 2016 at 18:37
  • $\begingroup$ I did not think a debate on the question remained. I assumed it was given the overall agreement on the Sapiens and Neanderthals question. I don't think there is a subjective threshold for how low should the fitness of the hybrid (unless we chose sterility). There is also no objective way to solve ring species problems. Also the concept of species based on reproductive isolation fits only sexually reproducing lineages. $\endgroup$
    – Remi.b
    Apr 11, 2016 at 18:46
  • 3
    $\begingroup$ I don't think we can comment about whether Darwin was aware of this problem, unless he documented it. Since we don't have a rigourous definition of species, we can parsimoniously assume that he didn't. In that case the other two questions would have no meaning. $\endgroup$
    Apr 11, 2016 at 19:14
  • $\begingroup$ @WYSIWYG I slightly rephrased to ensure the questions are not opinion-based. Does it seems better to you? $\endgroup$
    – Remi.b
    Apr 11, 2016 at 19:23
  • $\begingroup$ Yep but still it is quite easy to assume that Darwin didn't comment. If he did, we would probably have known. $\endgroup$
    Apr 11, 2016 at 19:27

1 Answer 1


The whole point of Darwin's theory was that transition from one species to another is extremely slow and gradual. There are plenty of quotes in "Origin of Species" stating this, and also affirming that there is no clear boundary between species and subspecies, or "races".

Quotes from Origin of Species > Variation under Nature (Chapter 2)

Quote 1

Nevertheless, no certain criterion can possibly be given by which variable forms, local forms, sub species and representative species can be recognised

Quote 2

Several experienced ornithologists consider our British red grouse as only a strongly marked race of a Norwegian species, whereas the greater number rank it as an undoubted species peculiar to Great Britain. A wide distance between the homes of two doubtful forms leads many naturalists to rank them as distinct species; but what distance, it has been well asked, will suffice if that between America and Europe is ample, will that between Europe and the Azores, or Madeira, or the Canaries, or between the several islets of these small archipelagos, be sufficient?

Quote 3

It is here the most definitive quote I managed to find and partially answers to your second question.

From these remarks it will be seen that I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience sake.

  • 3
    $\begingroup$ Great answer. +1 Thanks. I edited the post to change the format. Feel free to roll back if you don't like it. It'd be great if you could add the book/chapter/paragraph/page at which these quotes were taken. $\endgroup$
    – Remi.b
    Apr 11, 2016 at 22:09
  • $\begingroup$ Thanks, your edits made the response easier to read. As for the quotes, I gave a link to the book available online. There are no pages as such, but anyone may search it by text. These quotes are the first relevant ones it took me literally a minute to find. $\endgroup$
    – IMil
    Apr 12, 2016 at 8:42

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