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On Avian Subspecies: Buyer Beware? – Part 1

Editor's Note: The ABA Blog is excited to offer a four part series by renowned field ornithologist and researcher Steve N.G. Howell on the history and relevance of avian subspecific identification.  Steve is a senior international bird tour leader for WINGS and has written several books including A Guide to the Birds of Mexico and Northern Central America, Gulls of the Americas (with Jon Dunn), and the recently released and impressively received Petrels, Shearwaters, and Albatrosses of North America.  He lives near Point Reyes, California.


Part 1

            These days, birders are more interested than ever before in subspecies, and thus taxonomy. Now that we think we can identify most birds we see to species, we want to go further down the path, to put names on the variation we see in the species around us. But how realistic is this notion? And are subspecies a useful means of describing variation?

            I'd like to look at avian taxonomy from a philosophical and historical perspective, to help explain why subspecies are such a problem in modern ornithology, and by extension, in birding. The concepts are very simple, in theory. For better or worse, however, most of us live in a world of realities, not theories, and therein lies the paradox of taxonomy: if the world were simple enough for us to understand, we'd be too simple to understand it. Like the species concept, the subspecies concept might be viewed as a Procrustean bed in which Nature will not lie quietly – but as humans we have to try and make order out of chaos, one way or another.

            Any discussion of a subject should be based on definitions, so how might we define taxonomy, species, and subspecies?



            Taxonomy simply means classification, usually in reference to living things, such as avian taxonomy – the classification of birds. The units of taxonomy are taxa (singular: taxon) – for example, species and subspecies are different levels of taxa. Linked to taxonomy is nomenclature, the naming of things. It's basically human nature to put things in boxes – we are the taxonomic animal, and we need to classify things, to help make our world manageable.

            But what is a species? Basically, a species is impossible to define to everyone's satisfaction and Charles Darwin's statement of 1859 holds true today: "No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species." It's a bit like Justice Potter Stewart's famous "definition" of pornography: "I know it when I see it."

            Birders should recognize that taxonomists themselves can't agree on how to define a species. Books and countless articles have been written on speciation and species concepts, with sides taken and lines drawn between proponents of different species concepts. But while there may be an answer, for now it is unknowable, or at least indefinable. Still, as humans we need a working definition, even if it's imperfect, and here I'll use the biological species concept (or BSC). The BSC remains the foundation for the taxonomy of the American Ornithologists' Union (AOU) Committee on Classification and Nomenclature (often abbreviated by birders simply to "the AOU" or "they" – as in: "Where the %$&# did they put longspurs?").

            The classic definition of a biological species, as promoted by Ernst Mayr (1942) is basically: A population of actually or potentially interbreeding organisms, which are reproductively isolated from other such populations; the members of a species should be able to interbreed and produce fertile offspring. Within a biological species we can often distinguish variation. If this variation shows consistent patterns (beyond those associated with age and sex and season) it may be recognized by means of subspecies (often abbreviated as ssp.), which are designated formally by a trinomial, a third part of the scientific name.

            A bird's scientific species name is two-part and italicized, based on Latin, and comprises the genus name (Capitalized) and the species name (lower case). A genus can be defined simply as a group of closely related species, or of species sharing a common ancestor; it's another subjective definition, but one that lies outside the scope of this essay. For example, the Song Sparrow is known scientifically as Melospiza melodia, and shares its genus, Melospiza, with Lincoln's Sparrow and Swamp Sparrow, because these three species are considered more closely related to one another than to other sparrows. Song Sparrow populations breeding in the south arm of San Francisco Bay, California, are considered distinguishable from other populations, and are given the subspecies name pusillula, as in Melospiza melodia pusillula. Usually the population of Song Sparrow described first is given the same subspecies name as the species name, as in Melospiza melodia melodia, which is known as the nominate subspecies. This nominate designation is purely administrative, it does not imply that the subspecies melodia is any more typical than any other subspecies, or more widespread – simply that is was named first.

            A subspecies is synonymous with a race, despite the colloquial term "human race" – when really what people presumably mean is human species, of any race, creed, or color. Thus, for example, the race elegans means exactly the same as the subspecies elegans.

            But as with a species, how do we define a subspecies? Ornithology is the science of the study of birds. For something to be considered a science, certain criteria should be met. These include repeatability, whereby something can be repeated and confirmed by other researchers, using universally accepted measures that are considered unambiguous. Thus, in describing a subspecies (or species) the ideal is that anyone else can recognize it and identify it by its characters.

01 Isla San Benito Oeste, 5 Jul 1999 (1 of 8)-1Subspecies or species? Based on geographic location and season, in combination with bill size and shape, we can pretty safely say this Savannah Sparrow is of the taxon sanctorum, split in Handbook of the Birds of the World as a full species, San Benito Sparrow, but elsewhere considered a well-marked insular subspecies. Isla San Benito Oeste, Baja California, Mexico, 5 July 1999. © Steve N. G. Howell.

            In 1949, Dean Amadon attempted to formalize "the 75% rule" as the standard for defining a subspecies. That is, to be recognized as a subspecies, at least 75% of a population should be distinguishable from >99% of any other population, on the basis of a defining character or set of characters. This is a purely arbitrary definition, but at least it's a definition – with numbers. Applying it is not always easy, however, and this formalized definition was proposed after the great majority of subspecies had been described. Moreover, most descriptions of avian subspecies in recent years have not used the 75% rule but instead have used the concept of detecting statistically significant mean differences between populations, not on determining the extent of overlap in characters. (Whether statistical significance is the same as biological or evolutionary significance is a minefield I’m not going to enter here!)

            The AOU website (; accessed 4 April 2012) notes that "subspecies should represent geographically discrete breeding populations that are diagnosable from other populations on the basis of plumage and/or measurements, but are not yet reproductively isolated. Varying levels of diagnosability have been proposed for subspecies, typically ranging from at least 75% to 95%. Because subspecies represent relatively young points along an evolutionary time scale, genetic differentiation between subspecies may not necessarily parallel phenotypic divergence. … Described subspecies that represent points along a phenotypic continuum (cline) probably would not warrant recognition given further study."

            You'll note that this definition describes what the AOU would like subspecies to be, which is not how most subspecies were necessarily described. The AOU definition is also descriptive rather than prescriptive, and there remains no official template for describing subspecies. People do what they like, and it could be argued that avian taxonomy often falls short of science.

02 Rio Lagartos, Yuc (9 of 41)-1
Several subspecies have been described for both Red Knot (front left) and Short-billed Dowitcher (front center and right) – but these subspecies are rarely separable other than in breeding plumage, and then sometimes only with the aid of complicated statistics. Yucatán, Mexico, 1 December 2010. © Steve N. G. Howell.


Subspecies and History

            Humans like to collect things. Back in the day, especially throughout the 1800s, wealthy noblemen in Europe maintained collections of many things, bird specimens among them. They paid people to travel the world in search of new and novel additions to their collections. With specimens in hand, numerous new species of birds were described, usually based on the physical properties of structure and color – what has been termed the morphological species concept. The heyday of this naming lay in the 1830s to 1890s, and when something was named, its scientific name – and the person who described it – were enshrined forever; there was a certain prestige attached to the process.

            As one might expect, humans being humans, there was rivalry, both personal and international, in getting names published first; Nigel Collar (1999) discusses some good examples. Single specimens with scarcely any description might be published as new taxa, and because of the rules of taxonomy, these names usually had to stay. New names might even be rushed out the next day in newspapers to cement "priority" of naming in print, rather than wait for the slower process of possible peer review and publication in respected journals or more formal outlets.

            Virtually all subspecies diagnosis was based upon examination of specimens, that is, study skins made from dead birds. Other features, by which populations may be diagnosed, such as habitat, vocalizations, and breeding season, were rarely if ever used to define subspecies, yet such factors are often used in defining species. In essence, the subspecies developed as simply a morphological unit, one that could be identified in a museum tray by virtue of measurements and plumage. Perhaps paradoxically, some species appear indistinguishable by conventional measurements or plumage differences (think storm-petrels and owls, among others), and thus in a series of specimens they would not qualify as subspecies. Yet they can be distinguished as species by vocalizations, genetics, breeding seasons, habitat, and other criteria not apparent from a study skin!

            Subspecies were most often described based on males, which in many species are more colorful than females (and thus more favored by collectors), and often more conspicuous and easier to collect, such as when singing in the breeding season. However, subspecies may also be described based on distinctive features of the female plumage or even of a seasonal plumage. For example, the three widely recognized subspecies of Short-billed Dowitcher are best separated in breeding plumage – in nonbreeding plumage they are often not distinguishable. Thus, a subspecies may not be identifiable in all ages or sexes, or at all seasons.

03 Abbotts Lagoon, CA (29 of 152)-1
At least 11 subspecies of Common Yellowhthroat are recognized as occurring in North America, north of Mexico. Most are perhaps best distinguished in adult male plumage – so it is difficult (at best) to assign females and immature males, like this, to any subspecies when found outside the breeding season. Marin County, California, 31 August 2010. © Steve N. G. Howell.

            Following the publication in 1859 of Darwin's worldview-changing opus, the concept of subspecies came to have two meanings: one was as an incipient species, the other as evidence of the adaptive response of a species to local climatic conditions (Mayr 1982). With the philosophical change from a morphological to a biological species concept from the 1880s to 1920s, subspecies came to be seen as measures of geographic variation. (The present AOU definition of a subspecies, quoted earlier, indicates that this is still the case.) Scientists looked anew at all of the old "species" to see if some were simply subspecies of a more widespread and variable species. Consequently, many "species" were lost in this period: in North America alone, some 315 avian taxa that had been described as species were "lumped" – subsumed as subspecies into species that exhibited geographic variation (Mayr 1982). The formal term for this is synonymizing: when one subspecies is considered indistinguishable from another, the subspecies that was described later is a synonym of the earlier described subspecies, and is lumped with it; the earlier name takes precedence.

            As with many changes in fashion, the pendulum swung too far, and much of the indiscriminate lumping that characterized the 1900s to 1950s is now being slowly undone. For example, Murphy (1952) lumped seven former species of small shearwaters as subspecies of Manx Shearwater, which then became a species of worldwide distribution; no new data prompted this move, simply a change in philosophy. Nowadays, all seven of these taxa are split again as full species – and we have come full circle.

            Concurrent with the loss of many "species" was the description of many new subspecies, as ornithologists sought further examples of geographical variation. The resultant fad of naming subspecies saw its peak in North America from the 1890s to 1950s. Ideally, to describe a subspecies one would compare a series of fresh-plumaged specimens, of the same age and sex, with another series of the same makeup, but often this was not done. Many subspecies were distinguished based on slight differences, and rarely with adequate samples of birds in comparable plumage. For example, in 1934 Ludlow Griscom described a subspecies of Calliope Hummingbird from the species' wintering grounds in southern Mexico – on the basis of a single molting male in worn plumage!

            The 5th edition of the AOU checklist (published in 1957) was the last edition to list subspecies of North American birds, and their distributions; it doesn't tell you what the subspecies look like, however – for that you need to do a lot of digging in the literature and in specimen collections. Peter Pyle's two identification guides (1997, 2008) offer a starting point, but from his descriptions it is often difficult to infer what any given subspecies really looks like, or how easy it might be to distinguish in the field. For example, average color differences apparent in series of specimens are often expressed as "absolutes," as with Vermilion Flycatcher. For adult males, the Arizona subspecies flammeus is described as "head and breast … red to orange-red, often with pale mottling" whereas the Texas subspecies mexicanus is "deep red, without orange or pale mottling" (Pyle 1997:242). This sounds like a fairly clear difference. Yet when Brian Sullivan, Michael O'Brien, Chris Wood, and I laid out about 20 male specimens of each subspecies at the American Museum of Natural History in New York, we found only a subtle average difference in coloration, and many specimens could not be placed confidently with one subspecies or the other based simply on plumage tones.

            Implied but not stated explicitly in the 6th edition of the AOU checklist (published in 1983) is the notion that subspecies listed in the 5th edition had been evaluated critically (AOU 1983:xiii). The 1957 AOU checklist ostensibly marked the culmination of the subspecies era in North America. Subspecies description fell from fashion in the 1960s and 1970s, and the "glamour" in taxonomy is now with descriptions of cryptic species, and with revisions of genera or of families and other higher-level groups. Meanwhile, countless books and articles list subspecies as if they were inviolate and well-defined entities with mystical powers to elucidate geographic variation and distribution patterns. But are they?

 A complete Literature Cited section will appear at the end of Part 4…

Continue to Part 2

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  • Really great stuff- looking forward to the rest of this series!!

  • yes, a great topic for extended discussion; the concept of “species” is more ‘invented’ or mushy than most folks realize… and I’m still waiting to be convinced that the notion of ‘subspecies’ is biologically significant, and not merely defined into existence by human-perceived differences; perhaps Steve will persuade me…

  • Morgan Churchill

    Great essay, can’t wait for the rest of the series

    FYI, the proposal to split the Savannah Sparrow, including the San Benito form pictured above, has been posted again in the latest round of AOU proposals

  • J. Morlan

    Quoting cyberthrush, “the concept of “species” is more ‘invented’ or mushy than most folks realize..”

    Let’s consider the alternative. If there really are no such things as valid species, then all organisms would be reproductively compatible with all other organisms.

    Biological species simply map an order which is present in nature. The birds make the decision as to which species they belong by their choice of mates. We are merely observers, not inventors.

  • J.: the problem is that we can categorize our “observations” in a zillion ways… I can say that all humans with blonde hair and blue eyes are henceforth called Zikkers… and waaah laaah, Zickers have now been defined into existence (assuming I can give a consistent definition of “blonde hair” and “blue eyes” and “humans”).
    I’m not saying “species” is necessarily an invalid concept, just that its specific, consistent definition can be argued over endlessly. And to be technical, no one can prove that all Northern Cardinals are of the same species… perhaps the cardinal in my front yard can reproduce with the one in your yard, or, perhaps it can’t.

  • J. Morlan

    cyberthrush: Sure it’s easy to create artificial categories such as rocks of different sizes and call them by names. We can even invoke a sorting algorithm that has no intermediates and give big rocks a different name from small rocks. But that is not mapping natural reality.

    Consider the alternate hypothesis that Biological Species are NOT REAL. What predictions would that hypothesis make? It would be a world in which there were no reproductive barriers whatsoever; no mechanism to prevent free gene flow between any living beings. It would be rampant hybridization.

    But that is not the way of the world. Go out any day and make a list of the bird species in your local area. If biological species were not real, there would be no way to identify any birds because the boundaries would be a too sloppy to identify anything. Eventually natural selection would weed out all the diversity we enjoy and we would end up with only one “species” of bird.

    But once a new biological species has evolved, it has an independent evolutionary future because it is reproductively isolated. So natural selection acts on those individuals of each species independently from individuals of any other species. Natural selection is an intraspecies process and without the barriers to gene flow inherent in biological species there would be no diversity at all. Species are all that preserve biological diversity and without species there would be no diversity at all.

    Go back to your list of birds. Each species we record is the echo of a speciation event from the past. When we go birding, we are looking back into the history of life on this planet. We are seeing the consequence of speciation events every time we go birding. To pretend otherwise is to trivialize the reality and the power of the natural world.

    Subspecies on the other hand are merely a way of describing intraspecific geographical variation using nomenclature. Subspecies are not an evolutionary unit (unless they are also geographically isolated) and it is fair to argue that their boundaries are at least in part, the consequence of a human desire to create order.

  • Morgan Churchill

    My sense isn’t that cyberthrush is arguing that Species don’t exist (or at least are not a useful category), but rather they are not nearly as concrete or definite as some people think.

    Reproductive isolation is not black and white, it’s occurs on a continuum, a continuum which varies in space and time. That means that for many species, the determination using the BSC of whether a group of populations is one or multiple species is going to be a subjective decision

  • J. Morlan

    Sure species boundaries may be subjective when allopatric populations are analyzed. But in any given local area, there is a test of sympatry which determines unequivocally whether given populations are acting like species or not.

  • Morgan Churchill

    I think you would be hard pressed to find unequivocal used for species criteria by any ornithologists. Many (most) closely related taxa will breed when they come into contact. Check out all the recent research in the boreal zone, which is increasingly finding zones of hybridization which researchers did not realize existed until recently. Populations that come into contact and our closely related will often hybridize, but to what degree does the hybridization have to occur before you decide to lump or split?

  • Yes, Morgan has made the point perhaps better than I did… it’s not that ‘species’ don’t exist, it’s that their definition is fluid and imprecise — that’s why the AOU keeps forever changing bird species designations. Over time one might categorize species by visual observations and cues, or by structure/anatomy, or nowadays by molecular genetics, and each system will have different outcomes (and our limited understanding of genetics likely won’t be the final arbiter either).
    And again, assumptions are involved in presuming say a Blue Jay in Maryland is the same species as a Blue Jay in Michigan, based only on visual inspection — it’s almost a tautology (they’re the same species because they look the same; they look alike because they’re the same species). This all largely comes down to a debate between induction (which biology is full of) vs. deduction, in scientific empiricism, and not something resolvable here. And there are plenty of better expositions on the Web than I can make here on why “species” is an inexact or loose notion.

  • J. Morlan

    Of course “species” in inexact and there are many different species concepts which have been proposed. But we are talking here about “biological species” which are the basis of all avian diversity. Biological species are defined in terms of reproductive compatibility. But how does such a species concept relate to organisms that reproduce asexually? Of course, there it is basically useless and other species concepts (phylogenetic, evolutionary, morphological, etc.) have been proposed to deal with those.

    As for your Blue Jay in Maryland vs. a Blue Jay in Michigan, those are connected by a vast range in which there are other Blue Jays. Surely you are not suggesting that either of these populations is reproductively incompatible with populations in Ohio or Pennsylvania?

    In my neighborhood we have Steller’s Jays and Western Scrub-Jays. They are clearly behaving as valid species and it is perverse to continue to argue otherwise. However, my Steller’s Jays look different from Steller’s Jays in the Rocky Mountains. They are different subspecies. Could they actually be different species? Sure. But just because there are occasional hybrids between them, please do not argue that Blue Jay and Steller’s are not clear and well defined biological species.

    Throwing out a century of decided evolutionary and biological thought because there are some cases of evolutionary intermediacy is not useful.

    As for the term “unequivocal” let me leave you with this:

    Johnson, N. K. et al. 1999. Resolution of the debate over species concepts in ornithology: A new comprehensive biologic species concept. Pages 1470–1482 in Proceedings XXII International Ornithological Congress (N. J. Adams and R. H. Slotow, Eds.). University of Natal, Durban, South Africa.

    And as for the degree of hybridization before you lump or split, please check out:

  • Morgan Churchill

    Blue vs Steller vs Scrub Jay may not be the best example

    But what about Townsend’s vs Hermit vs Black-throated Green Warbler? All three hybridize in the areas they come into contact, and in the case of the Townsend’s vs Hermit, the Hybrid zone is changing. Are these 3 species? 1?

    What about Sapsuckers? Flickers? Fox Sparrows? Heaven forbid Gulls? all have dynamic hybrid zones with ornithologists not in common agreement over how many species are involved (and I am not talking about PSC versus BSC, but rather people who belief in BSC).

  • J.M.: sorry to drag this out (we’re really operating out of 2 different frameworks here), but I’ll try a different tact: imagine that I am an alien from the planet Volkar who knows nothing about the concept “species” 🙂 (on Volkar, Volkarians are the only living creature and there are no other ‘species’). I travel to Earth and from the state of Maine collect what you would call a “male Northern Cardinal;” from Virginia I collect another “male Northern Cardinal;” from Indiana a “female Northern Cardinal;” and from Georgia a “male Blue Jay;” and what the heck, from Tennessee I collect a raccoon!!
    Now give me a verbal definition of “species” by which I, as an alien, can consistently determine which of these 5 creatures before me are the same species and which are different species… I don’t think you can do it, except by making a great many assumptions that I, as a non-Earthling, wouldn’t have access to. (The best you might hope for is to describe some process of mixing all possible permutations of gametes together and seeing which combinations result in an embryo, but that wouldn’t be foolproof, nor work for the 2 male cardinals — or perhaps better yet, defining certain stretches of DNA as indicative of ‘same-speciesness’ (while still allowing for mutations I s’pose), but that would be a very cumbersome definition indeed to have to apply to every individual creature.) And my concern is how we classify “individuals” — you keep talking about “populations,” but you have to have a consistent (and non-circular) way of showing which “individuals” are truly members of any given population; as a practical matter we assume we know this all the time, but as an empirical matter it’s far more difficult to demonstrate — again, if I never see the Blue Jay in my yard reproduce with the Blue Jay next door (let alone with one 6 states over) how do I KNOW they’re the same species? Without witnessing every pairing up of Blue Jays in the wild, how do I know for sure whether there are 1 or 2 or 3 or… species of Blue Jays out there? I don’t.

  • J. Morlan

    Collecting specimens from different places will leave you unsure about which individuals belong to which population and it ignores the very idea of a population. Populations are defined in terms of their behavior. This is a very important point that you seem to be missing. It is social behavior which makes a population and social behavior which makes a species. Your visitors from outer space are at a huge disadvantage if all they do is collect scattered specimens and don’t bother to observe their natural history and behavior.

  • I won’t try to extend the impasse we’re at here, but for folks interested in such matters, Kevin Zelnio has done a decent job of summarizing some of the “species” issues in a new post for layreaders here:

  • Ted Floyd

    Steve Howell starts out by writing, “[A]s humans we have to try and make order out of chaos, one way or another.” And he goes on, “It’s basically human nature to put things in boxes–we are the taxonomic animal, and we need to classify things, to help make our world manageable.”

    I agree. That is to say, I agree with Steve’s observations about human nature. We sure do like our tidy concepts about how the world ought to work: the earth-centered solar system; space and time invariant and independent; Biblical inerrancy; and species.

    The first question we have to ask ourselves: Are species (never mind subspecies!) “real”? Oh, sure; they work great; they “help make our world manageable.” Species do a great job of cloaking the “chaos,” of meeting our perceived need to “put things in boxes.” But what if the chaos is really there? What if the boxes aren’t really there?

    Species do a dandy job of affirming our worldview. So it was, for millennia, with the geocentric universe and pre-Einsteinian physics. So it still is, for many people, with Biblical inerrancy. And so it still is, for many people, with species.

    But, again, are species “real”? That’s not a pie-in-the-sky question. Maybe it seems weird, but it would have seemed a lot weirder, as recently as the 19th century, to ask if space and time are “really” invariant and independent of one another.

    Speaking of the 19th century, it was the brilliant 19th-century thinker Charles Darwin who massively undermined the very concept (so Western, so Platonic, so Biblical, so constructed) of the “species.” Darwin realized, clearly and powerfully so, that the fundamental unit of selection is the individual. Natural selection, random events, mutations, etc., happen to individuals. (Or, according to a particular view, to those individuals’ genes.)

    Nearly all historians of science would agree that old “group selection” ideas have now been pretty thoroughly laid to rest. Of course, we can observe and predict patterns in assemblages of individuals, i.e., populations. And gene flow within and among populations is probably the key factor of interest for most contemporary evolutionary biologists. But what’s that got to do with “species”?

    The idea of the “species” is forcibly articulated in Genesis and Plato. Isn’t that enough to make us at least question whether “species” are for real?

  • Ted Floyd

    Do we have to have “species,” though, to prevent all individuals from being reproductively compatible with all other individuals?

    The Steller’s Jays in Boulder County, Colorado, would appear to be reproductively compatible with the Steller’s Jays in nearby Jefferson County, Colorado; that is to say, they appear to exchange genes freely. It may be the case, however, that these local Steller’s Jay populations (both from the foothills of the Rockies in n. Colorado) do not exchange genes freely with Steller’s Jays in the mountain ranges of the eastern Great Basin. There may be even less gene below between the population of “interior west” Steller’s Jays (Colorado’s Front Range, eastern Nevada’s mountains, etc.) and those breeding in coastal forests in the Pacific Northwest. Meanwhile, there is an apparently isolated Steller’s Jay population on islands off British Columbia; how much gene flow is there between that population and the Steller’s Jays of Boulder and Jefferson counties, Colorado?

    Oh, and what about Blue Jays? Both Blue Jays and Steller’s Jays occur in the foothills in Boulder and Jefferson counties. Hybrids have occurred, and are well documented. So there is gene flow between Blue Jays and Steller’s Jays.

    Seems to me that it would be a good idea not to worry about “species” limits in the jay genus Cyanocitta, and instead just try to describe quantitative differences in gene flow among the various populations of interest.

    As a wise man once wrote:

    “Certainly no clear line of demarcation has as yet been drawn between species and subspecies—that is, the forms which in the opinion of some naturalists come very near to, but do not quite arrive at the rank of species; or, again, between subspecies and well-marked varieties, or between lesser varieties and individual differences. These differences blend into each other in an insensible series; and a series impresses the mind with the idea of an actual passage.

    “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.”

    (That’s Charles Darwin, in The Origin of Species.)

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