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When speaking precisely or technically, one would say that "Homo erectus and homo sapiens are two species of hominid" rather than "Homo erectus and homo sapiens are two species of hominids." The hominid here should be singular because we are speaking about instances of a single class ("class" being used here in its broader sense, not in the sense of taxonomic grouping).

Now let's consider more common parallel constructions replacing the word species with other words.

Tide and Wisk are two brands of detergent.
Tide and Wisk are two brands of detergents.

Cats and dogs are two types of pet you can buy at Pets-R-Us.
Cats and dogs are two types of pets you can buy at Pets-R-Us.

I feel that using the singular noun to specify the class is more grammatically correct, but sounds stilted in conversation. Almost everyone I hear making these constructions uses the plural form. What are your thoughts about singular vs. plural here?

Edit

Here's a more extreme example that may help clarify the issue. Consider the following sentence:

It was my first attempt at calming an angry crowd, and I just stood there while people were hurling all kinds of insults at me.

Here I think the singular insult sounds strange, if not downright wrong.

Further Edit

I just noticed in A Treasury for Word Lovers (Morton S. Freeman, 1983) a section entitled "Types of Errors." This is a book about (American) English usage by an English professor and editor, which purports to be a "practical guide for serious writers and readers." I wish I could say this discovery satisfies the question, but in fact I now feel somewhat farther from the truth, if there is any single "truth" in this matter, although at least I don't have to worry about being wrong when using the plural form of the class in these constructions.

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I agree with your intuition about insult — sounds downright wrong to my US-English ear. – Kosmonaut Nov 24 '10 at 16:08
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@Kosmonaut: I wonder if in this case "all kinds of" acts as an informal adjectival phrase modifying insults. Remove the adjective and "people were hurling ... insult" is absolutely wrong. – Robusto Nov 24 '10 at 16:13
@Robusto: That sounds very plausible. Do you think that contrasts with your other two examples? – Kosmonaut Nov 24 '10 at 17:48
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I was thinking about your "detergent" example, and I think it doesn't sound as bad with the singular as the "pets" one. This is because of the fact that detergent can be countable or uncountable in certain situations, and the singular one definitely takes on that uncountable sense for me. – Kosmonaut Nov 24 '10 at 17:49
@Kosmonaut: Possibly. If I say "Smirnoff and Absolut are two brands of vodka," the singular vodka sounds completely correct and natural, and it would seem barbarous to substitute the plural vodkas there. – Robusto Nov 24 '10 at 18:03
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4 Answers

up vote 8 down vote accepted

This is an interesting question, particularly because of this dichotomy:

  • This is a type of apple. (not apples)
  • These are two types of apples. (not apple)

I think that the construction of the form "two types of apple" sounds more than stilted; it just plain sounds awkward, and I would be surprised that it sounds familiar and normal to anyone (at least speaking for US English).

The idea that a plural form would be used for a class is actually not strange at all in English. To express the fact that I like things belonging to the "apple" class, I would say:

  • I like apples.

I would not be able to use the singular to express this:

  • *I like apple.
  • *I like an apple.
  • *I like the apple.

Saying "I like apples" doesn't even imply that I am talking about multiple apples; one could say this, for example:

  • I like apples, although I've only ever had one in my life.

So, saying "I like all sorts of apples" seems to jive perfectly with the rest of English grammar.

This means that the strange case is actually this one:

  • This is a type of apple. (not apples)

Saying "this is a type of apples" is definitely not natural or familiar. It seems that, in phrases like "type(s) of X" ("kind(s) of X", etc.), there is generally number concord between the type-word and the class itself. Why that is, I don't know.

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There are definitely cultural differences then - 'These are two types of apple' is preferred over here. :) – CJM Nov 24 '10 at 15:42
+1 for some interesting points. Please see my edit to the question, which pushes the boundaries further. – Robusto Nov 24 '10 at 15:56
Awarding you the check. See my final edit to the question as to the reason. – Robusto Nov 25 '10 at 17:25
up vote 4 down vote

I agree that the singular is not only correct, but I also think it sounds far better too.

Cats and dogs are two types of pets you can buy at Pets-R-Us.

I actually stumbled over the this example; it just doesn't feel right when I say it.

[PS I'm British - I'm not sure if there are some cultural variations here]

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+1 and thanks for the input. I'm sure there are cultural variations. The dilemma I refer to concerns what I hear in the U.S. vernacular. – Robusto Nov 24 '10 at 15:28
up vote 2 down vote

The best way I can think of to explain the correctness of the singular form as opposed to the plural is by way of this example.

I would say

Greyhound and Boxer are both dog breeds.

I would certainly not say

Greyhound and Boxer are both dogs breeds.

Therefore why should I say

Greyhound and Boxer are both breeds of dogs.

To me it sounds equally silly but that's because I'm a rather analytical creature.

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This logic doesn't hold up under scrutiny. "Gears and axles are both machine parts", but not "Gears and axles are both parts of machine". – Peter Shor Oct 10 '12 at 12:55
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No, that would be 'parts of a machine' but that doesn't affect the plurality of it. – Alan Gee Oct 10 '12 at 12:59
There is really a difference here between talking about types/breeds/kinds of things (as referred to in the question) and talking about parts of machines. With the former we can say that the dog is a a thing of which there are many breeds. If we do the same with the machine and its parts it doesn't quite have the same meaning does it? – Alan Gee Oct 10 '12 at 13:08
up vote 1 down vote

There are many discussions on this type of things, such as "kind(s) of book(s)", "kind(s) of drink(s)", etc. I read quite a few threads about this on StackExchange.com and I found there were no clear answers. Finally I had to search "type(s) of thing(s)" as a whole keyword in Google and got the following results:

  • "types of thing" — 474,000
  • "type of things" — 18,700,000
  • "types of things" — 37,000,000
  • "type of thing" — 86,300,000
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