Types of things vs. types of thing
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 jibe 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.
This may not not be strictly grammatical, but as a point of logic, I interpret it like this:
"Types of thing"
Here you're talking about types as belonging to a single grouping category. The concept is one of making a union.
"Types of things"
Here you're talking about a number of forms of a number of different categories. Or you're talking about the things, plural, first and foremost, and types is a descriptor to indicate diversity.
You'll note these are distinctly different descriptions for different ends.
Examples:
"Frogs and cows are two different types of animal."
This is stressing the belonging of frogs and cows to one category.
"He was throwing all kinds of insults at me"
Many insults were thrown, not one. The stress on the insults - plural - the insults are of many different kinds. The intent is not to show belonging of all insults to one type, and so the notion of many insults is retained through the plural.
In more detail
In rough "linguistic speak" it's like this:
"Frogs and cows are two different types of animal."
The intention is unite two things under one category. In grouping them, we're showing belonging - belonging to a singular animal. In describing that one group to which both sub-types belong, we only need the singular. Frogs and cows are grouped to be labeled; a single 'notional object', if you like, under the word types. animal is a singular description for a that singular group: types.
"He was throwing all kinds of insults at me"
The intention is to describe the insults, plural. insults is a plural form of a 'notional object' within the sentence; the grouping of all kinds is to qualify the plural insults. It needs to be plural.
As far as I can see, this observation makes sense of anything I can throw at it with my Australian ear. I have to admit, however, the 'ears' of the general public are probably changing. "Kinds of things" is being used so often of late in place of what I believe should be "kinds of thing" (being consistent with the above logic). I'm sure many people will hear it another way. However, I should point out that there is, at least, internal logic in considering the true object to which number agreement should be made regarding the theory above... And that logic will never (well, not to my ear anyway) create a truly awkward sounding sentence... not that I've thought of thus far...
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]