The classification of living things depends upon the literature. Access to this literature is essential to taxonomic research and to our understanding of biodiversity. There have been tremendous efforts to digitise the world’s biodiversity literature; the Biodiveristy Heritage Library (BHL) alone has uploaded over 54 million pages, all of which is freely accessible online. Our scientific literature is far more accessible than it has ever been, but that does not mean it is easily discoverable. Much of the taxonomic literature online remains outside the linked network of scholarly research. But that is rapidly changing.<br> Taxonomic aggregators are an invaluable source of authoritative information on species names and their hierarchical classification. It is critical that this information includes citations for taxonomic descriptions, that these citations link to the published literature online and that (wherever possible) the citations include DOIs (Digital Object Identifiers). The DOI is an essential part of a publication’s bibliographic metadata and should be included (as a live link) in any reference to that content.<br> However, the definitive (DOI’d) versions of recent publications are frequently behind paywalls. And, while much of the historic literature available online is open access, commercial publishers are uploading out-of-copyright publications onto their own websites, assigning DOIs to “their” definitive versions (the versions that must be cited in other publications, as per DOI requirements) and then locking the defintiive versions behind paywalls. This is perfectly within their rights. DOIs may be assigned to legacy publications retrospectively, providing that: a) the party assigning them owns the rights for the content, or has permission from the rights holder to assign a DOI, and b) the publication does not already have a DOI. If there are no rights attached to a piece of content, anyone can assign a DOI to it.<br> This means that citation traffic from the bibliographies of current publications is increasingly directed towards commercial publishers’ websites, rather than towards open access versions, such as those freely available on the Biodiversity Heritage Library (BHL). However, taxonomic aggregators are not bound by the same obligations as publishers and may therefore choose to link to any online version of a publication (although the DOI should still be included in the citation).<br> Many taxonomic aggregators link to the literature available on BHL. The taxonomic name profiles in EOL (Encyclopedia of Life), GBIF (Global Biodiversity Information Facility) and ALA (Atlas of Living Australia) each contain a BHL bibliography: a list of links to the pages in BHL that contain an identified mention of that taxon name. However, the lists of returned results can be long, and they may or may not include the citations for accepted names, synonyms and taxon concepts. Some biodiversity aggregators feature these key citations on the names pages (or tabs) of taxon profiles. However, where these do exist, they are usually plain text rather than links.<br> BHL is now registering DOIs for the content it hosts and is creating landing pages for articles, containing the full bibliographic metadata, including (where applicable) the DOI. Articles are now discoverable by article title, keywords within titles (scientific names, locations, traits, etc.), author names and DOIs, and can be easily linked to (via their landing pages) by other parties.<br> This paper will examine the issues, benefits and complexities associated with linking to definitive versions, the difference between easy and open access, the ethics of putting out-of-copyright content behind paywalls, and the future of creating order amongst the massively expanding resource of literature online.
