But just because their habitats have changed and their numbers proliferated, doesn't mean they are an entirely new species. Rather, "obsessive compilers," as Loveland and Reacle call them, have been around for a long time -- at least since Pliny the Elder wrote his 37-volume Natural History in ancient Rome. Over the centuries, obsessive compilers have been behind not just encyclopedias, but dictionaries, medical texts, histories, and even object collections, such as herbaria. Loveland, of the University of Cincinnati, and Reagle, of Northeastern, point to examples such as Louis de Jaucourt, who wrote some 25 percent of Denis Diderot and Jean Le Ron D'Alembert's Encyclopedie and James Tytler, who assembled the second edition of the Encyclopaedia Britannica. Today's Wikipedians may have less visibility than the compilers of the past, but they are motivated by the same compulsive urge to collect knowledge and organize it.

Another similarity between encyclopedias of old and Wikipedia: the process of building upon existing work bit by bit, or what the authors call "stigmergic accumulation." Now, that's a mouthful, but it's also a great metaphor. "Stigmergy," they write, describes "how wasps and termites collectively build complex structures by adding to the product of previous work rather than by communicating directly among themselves."

We tend to think of older encyclopedias as independent works, created by their authors and editors afresh in pursuit of greater depth and accuracy. Not quite, the authors explain. In a time before copyright protections, reference works were rife with "borrowing." Scottish "pirates" reprinted Ephraim Chambers' 1728 Cyclopaedia in its entirety, which itself borrowed heavily from the Dictionnaire de Trevoux. "Indeed," Loveland and Reagle write, "Chambers avowed that the Cyclopaedia contained 'little ... new, and of my own growth." Men like Chambers were always a bit author, a bit compiler, a bit borrower, a bit editor. If Wikipedia complicates the notion of "authorship," it's not as though that notion were ever simple to begin with.

Even Wikipedia's open ideology has antecedents during this period. Diderot announced that people were free to reuse the art from his Encyclopedie -- "a stance," the authors note, "probably meant to justify his and his colleagues' appropriation of illustrations from the 'Description des arts et metiers.'"

Perhaps most of all, the originality of Wikipedia's collaborative approach is suspect. Massive works of reference almost by their nature require the input of many people. Diderot's and D'Alembert's Encyclopedie had around 140 different specialists make contributions. The Oxford English Dictionary is famous for its collaborative process, to which thousands of people contributed slips of paper noting the appearance and context of unusual words.