Is "one" unnecessary in this quote of Melville?

Here, one serves as a pronoun whose antecedent is classification.

Here’s a simple example:

• Which bag would you like?
• I’d like the blue one, please.

Adding a comma between the descriptor and the pronoun doesn’t maintain the sense and may make the whole sentence ungrammatical.

In my simple example, simply removing the word “one” doesn’t work either. In your quote, however, “outline” can be treated as a substantive, so “one” may be deleted as you suggest.

The intent of the original is a desire to have a classification, even one presented merely as an outline.

This answer is intended as an addition to the existing discussion; it does not repeat the relevant information already provided, but queries some of the points made.

The OED cites fairly extensive use for outline in compound form:

outline drawing n.

1850 J. Leitch tr. K. O. Müller Ancient Art (new ed.) §74 Outline drawing and monochrome painting.

outline map n.

1836 in N. Amer. Rev. Jan. 258 An Atlas of Outline Maps; intended for the use of Students in Geography. 1911 Chambers's Jrnl. Jan. 79/2 The outline maps of the arena are so manipulated that it seems to fly past you. 1982 P. Fitzgerald At Freddie's iv. 30 Hannah..gave out some outline maps..on which the children were to fill in the capitals of Europe.

outline sketch n.

1835 Knickerbocker 6 63 I present an outline sketch of one of that species of the genus homo..which Custom has christened with the expressive appellation of Loafer! 1865 J. Lubbock Prehist. Times vi. 188 The facts already ascertained..supply us with the elements of an outline sketch. 1992 Artist's & Illustrator's Mag. May 20 Once I am happy with the outline sketch, I then roughly shade and mark out wrinkles.

outline plan n. a draft or sketch plan lacking details.

1850 Philos. Trans. (Royal Soc.) 140 169 Outline plan carefully constructed from the heart of an embryo, measuring 5 inches from the vertex to the coccyx. 1972 Guardian 5 Aug. 11/2 Westminster City Council are sitting on outline plans to knock down..a tenement of 89 flats. 1986 I. F. Roberts & L. M. Cantor Further Educ. Today 156 The decision to establish institutes of higher education..did not materialize wholly in the form proposed in the outline plan.

Melville uses ‘outline’ in a manner, similar to ‘outline classification’, in one other instance:

Of the Right Whale, the best outline pictures are in Scoresby; but they are drawn on too small a scale to convey a desirable impression. He has but one picture of whaling scenes, and this is a sad deficiency, because it is by such pictures only, when at all well done, that you can derive anything like a truthful idea of the living whale as seen by his living hunters.

Importantly, Melville uses ‘one’ after a modifying noun (Wiki) in one other instance also, though the full expression is more complex:

It was a humorously perilous business for both of us. For, before we proceed further, it must be said that the monkey-rope was fast at both ends; fast to Queequeg’s broad canvas belt, and fast to my narrow leather one. So that for better or for worse, we two, for the time, were wedded; and should poor Queequeg sink to rise no more, then both usage and honor demanded, that instead of cutting the cord, it should drag me down in his wake.

To provide some context on the use of ‘one’ after a modifier in Moby-Dick, I include a few other examples below. Altogether, Melville used ‘one’ after a modifier just over 30 times, in a work that consists of around 210 thousand words and in which 'one' was used approximately 900 times.

For a perspective on whether this usage was out of the ordinary or is less frequent now, it is necessary to examine the same numbers, and some others, across other authors and time periods. Without such an analysis, an evaluation on whether this usage is at all unusual is not possible.

In Fowler's view, discussed in P. P. S., this usage is "established idiom".

They say that men who have seen the world, thereby become quite at ease in manner, quite self-possessed in company. Not always, though: Ledyard, the great New England traveller, and Mungo Park, the Scotch one; of all men, they possessed the least assurance in the parlor. But perhaps the mere crossing of Siberia in a sledge drawn by dogs as Ledyard did, or the taking a long solitary walk on an empty stomach, in the negro heart of Africa, which was the sum of poor Mungo’s performances—this kind of travel, I say, may not be the very best mode of attaining a high social polish.

No town-bred dandy will compare with a country-bred one—I mean a downright bumpkin dandy—a fellow that, in the dog-days, will mow his two acres in buckskin gloves for fear of tanning his hands. Now when a country dandy like this takes it into his head to make a distinguished reputation, and joins the great whale-fishery, you should see the comical things he does upon reaching the seaport.

The four whales slain that evening had died wide apart; one, far to windward; one, less distant, to leeward; one ahead; one astern. These last three were brought alongside ere nightfall; but the windward one could not be reached till morning; and the boat that had killed it lay by its side all night; and that boat was Ahab’s.

P. S. In addition to the sentence that is being discussed, that includes the expression 'outline one', Melville uses 'outline' on two other occasions. Although those usages are of a different kind grammatically and refer to a particular style of picture, their meaning may suggest (of one of them in particular - “mechanical outline of things”) that ‘outline classification’, as an expression, would have been quite possible, in Meville’s usage; this has already been shown, from a different point of view, as likely in Giles' answer and Sven's comments; additionally, Melville does use 'one' after a noun modifier.

Lastly, if we were to consider the question of whether 'one' is necessary in that sentence not just from a grammatical but broader point of view, we probably could say that, in the sentence, Melville's voice remains authentic. However, this is somewhat subjective without a more detailed examination.

Then again, there is an imposing quarto, written by one Captain Colnett, a Post Captain in the English navy, entitled “A Voyage round Cape Horn into the South Seas, for the purpose of extending the Spermaceti Whale Fisheries.” In this book is an outline purporting to be a “Picture of a Physeter or Spermaceti whale, drawn by scale from one killed on the coast of Mexico, August, 1793, and hoisted on deck.”

For the most part, the English and American whale draughtsmen seem entirely content with presenting the mechanical outline of things, such as the vacant profile of the whale; which, so far as picturesqueness of effect is concerned, is about tantamount to sketching the profile of a pyramid.

P. P. S. Some observations on the grammatical function of 'one' in the sentence discussed (Following a question in a comment below)

From Fowler's Modern English Usage (Second Edition):

One as a 'prop-word' is a name given by grammarians to the use of one or (ones) to support an adjective or other qualifying word or words that would be awkward or ambiguous standing alone.

The second resolution was the US one./The satellite was a small test one.

This is established idiom, but should not be employed unnecessarily. It could perhaps be argued that, even when not needed to remove awkwardness or ambiguity, one or ones may be justified as contributing a subtle emphasis: that His life was sedentary and lonely one gives sharper picture than His life was sedentary and lonely...

Melville's usage of 'outline one' does create an emphasis.

The OED (and Merriam-Webster; I will focus on the OED as it is more detailed in this particular regard) describes one in the capacities of an adjective, a noun and a pronoun.

There are numerous cases of usages in each group.

In the group of pronoun use, the OED cites:

V. As substitute for a noun or noun phrase.

13. Following a determiner such as the, this, that, yon, any, each, every, many (a), other, such (a), what (a), what kind of (a), which, or (in certain phrases) following a, or (from Middle English onwards) following an ordinary adjective (occasionally also a noun used attributively) preceded by any of these or (in plural) alone.

1799 A. Young Agric. County of Lincoln 194 There was a horse-pasture and a sheep one contiguous.

1815 J. Scott Visit to Paris xi. 238 Of all the practicabilities, which at present offer themselves to that country, the one that is most [promising] is the stability of the government of the Bourbons.

a1864 Ld. Tennyson Poet's Song 14 The nightingale thought, ‘I have sung many songs, But never a one so gay.’

1868 E. A. Freeman Hist. Norman Conquest II. App. 604 There is no reason to think that the pilgrimage was other than a self-imposed one. 1875 H. J. S. Maine Lect. Early Hist. Inst. xii. 342 The examination of new materials and the re-examination of old ones.

1881 F. J. Britten Watch & Clockmakers' Handbk. (ed. 4) 67 Drawing out the quarter screws of the balance nearest the fast position..and setting in the ones nearest the slow position.

1911 Encycl. Brit. XXVI. 29/2 The shaping machine does for comparatively small pieces that which the planer does for long ones.

1953 H. Mellanby Animal Life in Fresh Water (ed. 5) xi. 229
Water-snails are similar in appearance to the familiar land ones.

1956 J. C. Powys Brazen Head (1969) i. 12 She waved her hand, the one that wasn't being used to prevent his getting up.

14. Following a determiner or adjective (as in sense C. 13), without contextual reference: a person having the characteristics indicated.

1973 M. P. Holt & D. T. E. Marjoram Math. in Changing World ii. 21
There seems to be evidence for an evolution of intelligence from Homo faber, the tool-user, to Homo sapiens, the wise one.

15. a. A person or thing of the kind already mentioned. Also (Irish English (northern)) in plural. Formerly also used pleonastically or emphatically at the end of a clause or sentence.

1983 M. Roberts Visitation v. i. 158 She begins to recognise this landscape, one she has visited before.

However, the OED does not regard this class of pronouns as indefinite.

(This may need further examination, and so to be continued...)

One is a pronoun, referring to the classification. It's the center of the noun clause “an easy outline one”, in which both easy and outline qualify one.

While older grammars classify one as a pronoun, modern grammars tend to classify it as an anaphoric common noun. This simplifies the rules around the usage of pronouns, because if one is classified as a pronoun, it's unusual in that it can have a modifier. However the meaning of one that is applicable here is listed under “One (pronoun)” in most dictionaries.

Here's what Melville is saying, broken down into shorter sentences:

1. Now the various species of whales need some sort of popular comprehensive classification.
2. For the present, the classification will be an outline classification, which is easy.
3. Hereafter, subsequent laborers must fill in the outline classification in all its departments.

The expression “outline classification” is a set phrase with a well-established technical usage. Contextually, it seems to mean a classification that is presented as an outline, that is, a hierarchical presentation of categories, their subcategories, their subsubcategories, etc., without significant details regarding the content of the (sub…)categories. (Programmers and computer scientists would call this a tree structure.) Here are some usage examples:

• (espistemology, ~1902) Charles Sanders Pierce's classification of the sciences. Pierce published several classifications of the sciences, some of which are termed “outline classification”. In his Principles of Philosophy, the part about the classification of the sciences starts with a chapter titled “An Outline Classification of the Sciences”, where he breaks down science into fields, then breaks down the fields into subfields, and so on, until he reaches categories that he described in one sentence at most. The net chapter “A Detailed Classification of the Sciences” goes into more detail about the process of classification. Here is a modern graphical presentation of this classification by Tommi Vehkavaara.
• (zoology, 1983) An Outline Classification of the Phylum Nematoda. Australian Journal of Zoology 31(2) 243–255.
• (botanics, 1982) “An outline classification of plants”, in An Introduction to Plant Taxonomy by Charles Jeffrey.
• (linguistics, 1992) “Outline Classification of the Indoeuropean Languages”, in An Indoeuropean Classification: A Lexicostatistical Experiment by Dyen et al.

It's somewhat uncommon to use a noun qualifier with one, but it can be done, especially when there's also an adjective. Here are examples that don't even have an adjective:

Although the planning system is the same regardless of whether a site is in an urban location or a countryside one, …
some people wrap that pig up in a pastry blanket, rather than a bacon one
Even though there are other types of showers (…), a tile one is a great option

There is a phenomenon of anaphoric reference that makes it remote and so somewhat detached from the referent - many of the examples given in reply have this quality.

Now the various species of whales need some sort of popular comprehensive classification, if only an easy outline one for the present,[...]

In Melville's quote this is happening to a small degree. "One" could be replaced by the referent "popular comprehensive classification" or just "classification" and our memory will be eased by the repetition of the actual referent; or "one" could be eliminated altogether so that there is no anaphoric reference to exercise our memory.

I earlier said that there is a slight detachment of the anaphor: it is caused by the word "easy" qualifying "outline". Using "easy" causes a bit of a hiccup in our interpretation because what else should an outline be but easy - "easy" used here is a pleonasm. If "easy" is eliminated then "one" is less remote, and if both "easy" and "one" are eliminated there is no loss in understanding and our memory is less exercised.