According to the Wiki page for binomial nomenclature, we are supposed to capitalize the first word when naming species regardless of where it occurs in the sentence. To me, this seem very incongruous with not only the English language but all languages that I know of which use Latin characters. So what is the justification for this (if any)?

For example, I have a house cat but my biology teacher has a Felis catus.


I see where people may be having trouble with this now. I don't know if this is a rule or not but I can find no other real examples of capitalization being used like this. virmaior has shown me that there is a seldom used rule about Platonic forms which would apply if creatures where forms. That is not the case (they have actual bodies and so on) but perhaps that really is the best answer as incorrect as it may be.


Oldcat has brought up an interesting point below:

Felis is the proper name of the genus, not of any particular cat. We do capitalize breeds of cats, like Persian and American Curl, because they are the proper name of the group.

This is similar to points made by a few others and may be related to Platonic forms. I'm still not seeing what would make the less specific term Felis catus capitalized while the more specific term catus by itself isn't (aside from the seemingly arbitrary explanation that Felis catus is a Platonic form while catus alone isn't)? Unfortunately, the fact that we then again begin capitalizing the even more specific breed name only confuses the issue further. Do you see my point?


It occurs to me that there may be multiple answers to this. In scientific technical writing it could be proper to use Felis catus and in common usage it may be more proper to simply use felis catus (or possibly Felis Catus but not Felis catus because that would be a ridiculously arbitrary rule for a common language). It isn't lost on me that a very similar idea has been mentioned several times but the difference is that here I'm accepting that these terms have, in fact, become part of the English language while rejecting binomial nomenclature as part of the English written language. Is that a common consensus? If it is then shouldn't there be a general rule about this instead of a bunch of specific cases where groups are considered proper nouns in some cases and improper nouns in others?

  • 3
    iczn.org should really be your first port of call. They make the rules; nothing to do with rules of English.
    – Frank
    Jul 2, 2014 at 11:26
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    I am not sure how something the English language actually does can be very incongruous with the English language. Writing fElIs cAtUs, now that would be very incongruous.
    – RegDwigнt
    Jul 2, 2014 at 12:29
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    @krowe Reg's point is that if users of language X exhibit behavior Y when using X, then behavior Y is by definition not incongruous with the rules of language X—indeed, behavior Y is necessarily part of the rules of language X
    – nohat
    Jul 2, 2014 at 15:25
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    It is just jargon. The genus name is a proper name, like Bob. It isn't a shock that Bob is capitalized.
    – Oldcat
    Jul 2, 2014 at 17:19
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    @Oldcat I thought the same thing initially, but upon further reflection realized that it doesn't really answer the question. We don't capitalize cat. It's not a proper noun. But Felis domesticus is capitalized. Why would the scientific designation suddenly make it a proper name? How does "My cat purred" differ from "My Felis domesticus purred"?
    – Kit Z. Fox
    Jul 2, 2014 at 18:41

4 Answers 4


While it's not really a matter of English choice since it's decided by international standard (see Frank's comment above), I think the base justification was originally a philosophical statement, viz., that a species is a Form (where Form here means Platonic form or Aristotelian essence). See for instance here.

By convention, in philosophy, these were/are capitalized -- a capitalization rule that was once common in English (e.g., capitalizing Justice when it refers to what we might now call justice proper or justice itself). You can see this usage in Locke (though it also is an oddity in English capitalization).

  • 1
    You still see this kind of thing in modern English, with less lofty Ideas, especially in Marketing, or discussing certain topics or areas of study, such as Chemistry or Physical Therapy (even when they do not refer to the proper name of a university course). (Let me be clear that I'm not arguing that this is correct, just reporting what I see.)
    – John Y
    Jul 2, 2014 at 12:07
  • Fair enough. I do capitalize (at least on a resume) Chemistry and Philosophy (for my undergraduates) and so on for ... the other letters that come after my name
    – virmaior
    Jul 2, 2014 at 12:15
  • I still don't like it but since no one has a better answer as to why. I went ahead and accepted this.
    – krowe
    Sep 8, 2014 at 16:56
  • @krowe it's just a convention. Why do we or don't we capitalize anything? Why do Germans capitalise all nouns? Why do the French capitalize titles the way they do? Why do we even distinguish between lower and upper case?
    – phoog
    Sep 26, 2015 at 6:00

Basically, all taxa down to and including genus (but no further) are considered proper nouns, and thus are capitalized.

However, only Genus species must be written in an italic Latin script; higher taxa aren’t italicized.

Even papers written in other scripts than Latin (say, in Greek or Cyrillic, or in Chinese or Japanese) are expected to switch to Latin italics for binomials. This rule is in the standard that biologists use for these things.

It’s when you get to talking about genes that things get weird, because the rules for italics differ between animal and plant genes. Strangely enough.

  • Your last point makes me wonder what convention would apply when a plant gene is spliced into the chromosome of an animal, and vice versa. And what about organisms that belong to neither the plant nor the animal kingdom, such as blue-green algae or fungi? (I'm not disputing your point, merely suggesting that standardizing on a single convention for all organisms would be the most logical way to handle the issue.)
    – Erik Kowal
    Dec 3, 2014 at 9:05

The case orthography of binomial nomenclature (capitalized genus name, uncapitalized species name) dates back to Linnaeus' Systema Naturae (Google Images: "linnaeus primates"), though I'm not sure if he ever rationalized this choice. We preserve the Linnaean tradition.

But what's more important is to see BN as foreign to English. It's a modern international "language", that is not part of English. We can incorporate it into English (as can be done in any other language), but then are supposed to italicize it to emphasize its foreignness. Felis for example is not an entry in the OED, though some genus names have lately entered English, but only upon converting to uncapitalized roman type. BN is foreign for its own sake, not simply because its words are Latinized or from Greek or Latin roots. A language with no native speakers (except biologists?).


According to the conventions of biological nomenclature--zoological or botanical or other--both the genus and species (and subspecies if any) of a binomial name is italicized, the first letter only of the genus is capitalized, and the species is not capitalized. Thus Felis catus is the appropriate way to refer to a domestic cat--in scientific terms. Likewise we have Homo sapiens for humans. If you don't have italics available, you would still want to capitalize the genus, thus: Felis catus or Homo sapiens. Biological names are written in Latin form. Formerly they were in Latin (as Homo sapiens is), but many more recent names hardly even look Latin (e.g. Ogasawarana ogasawarana, a species of Japanese land snail, named for a place in Japan). "Higher" divisions of living things are not italicized. Thus we would say Kingdom Animalia, phylum Chordata, etc. It is common in biological nomenclature to identify the person who first gave the scientific name by appending his/her name to the genus and species; but the name of the biologist is not italicized, and may be abbreviated. Thus Homo sapiens (L.) or (Linn.) or (Linnaeus). Rules for zoological nomenclature are found in the International Code of Zoological Nomenclature (other Codes exist for botanical and other nomenclatures), which can be found on line.

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