The Quinarian system was a method of zoological classification which had a brief period of popularity in the mid 19th century, especially among British naturalists. It was largely developed by the entomologist W. S. MacLeay in 1819. The system was followed by Nicholas Aylward Vigors and William John Swainson. Swainson's work on ornithology gave wide publicity to the idea.[1]
Quinarianism gets its name from the emphasis on the number five: it proposed that all taxa are divisible into five subgroups, and if less than five subgroups were known, quinarians believed that a missing subgroup remained to be found.[1]
Presumably this arose as a chance observation of some accidental analogies between different groups, but it was erected into a guiding principle by the quinarians. It became increasingly elaborate, proposing that each group of five classes could be arranged in a circle, with those closer together having greater affinities. Typically they were depicted with relatively advanced groups at the top, and degenerate forms towards the bottom. Each circle could touch or overlap with adjacent circles (a phenomenon called 'osculation').
Another aspect of the system was the identification of analogies across groups:[2]
[W]e shall consider that to be a natural system which endeavours to explain the multifarious relations which one object bears to another, not simply in their direct affinity, by which they follow each other like the links of a vast chain, but in their more remote relations [analogies], whereby they typify or represent other objects totally distinct in structure and organization from themselves—Swainson, 1835:197[3] (italics in the original)
Quinarianism was not widely popular outside the United Kingdom; it had become more or less unfashionable by the 1840s, during which time more complex "maps" were made by Hugh Edwin Strickland and Alfred Russel Wallace. Strickland and others specifically rejected the concept of "analogy".[4] These systems were eventually discarded in favour of principles of classification based on evolutionary relationship.[1][5]