For example, Jack Horner is considered a lumper because he claims that Stygimoloch and Dracorex (and no doubt others) are merely different growth stages of Pachycephalosaurus. At the opposite end of the scale are the "splitters" (the most extreme of which is Bob Bakker) who tend to ignore similarities and magnify differences, even ones which could be due to a creatures age or sex.
The earliest tandem use of the terms "lumper" and "splitter" was by the biologist George G. Simpson in his 1945 work "The Principles of Classification and a Classification of Mammals." As he put it, "splitters see very small, highly differentiated units – their critics say that if they can tell two animals apart, they place them in different genera … and if they cannot tell them apart, they place them in different species. Lumpers, on the other hand, see only large units – their critics say that if a carnivore is neither a dog nor a bear, they call it a cat."
References
George Gaylord Simpson (1945) "The principles of classification and a classification of mammals." (Bulletin of the American Museum of Natural History)
