272 I The Process of Evolution 



HIGHER CATEGORIES 



Much attention is given in the hterature to the subject loosely de- 

 scribed as the "origin of higher categories." In the strictest sense, 

 this is a problem for the psychologist, for higher categories ( Reptilia, 

 Lepidoptera, Rosaceae, Basidiomycetes) are, like species, concepts, 

 not things. Their origin is shaped by many often unsuspected factors, 

 such as preconceived systems of analysis, language structure and 

 usage, and others. (See Chap. 13 for a discussion of this problem.) 

 Nevertheless, the concepts stand for something, and it is pertinent 

 to inquire into the causes of the patterns commonly observed in 

 nature. 



It seems likely that major discontinuities of the fossil record are 

 a result of accidents of sampling. If a group enters a new adaptive 

 zone during a period of tachytely, the chance of fossilization during 

 the brief transition period is reduced simply because of its brevity. 

 There may be gaps in the foliage of a tree, but even a phylogenetic 

 tree has continuous branches. The category system of Linnaean tax- 

 onomy has somewhat magnified the size of apparent discontinuities. 

 Rare organisms, more or less intermediate in characteristics between 

 major clusters, are generally arbitrarily assigned to one or the other 

 large cluster for the sake of convenience. Thus monotremes are 

 lumped with mammals, micropterygids with moths, and fungi with 

 plants. At relatively low taxonomic levels, reticulate evolution leads 

 to complications. If a new species of plant is the amphidiploid hybrid 

 derivative of two other species in different subgenera, it is often 

 described as a member of an existing subgenus rather than as the 

 sole member of a third one. 



As noted earlier, taxonomic higher categories are in part the 

 result of extinction which produces morphological discontinuities. 

 This is a subject with which it is difficult to come to grips, and it has 

 received only superficial attention from biologists. It is related to the 

 matter of competition previously discussed. Specialized groups, when 

 interacting with primitive groups, have often proved more success- 

 ful, with the eventual disappearance of the more primitive types. 

 The comparison of primitive and specialized here implies no judg- 

 ment of quality; two groups are compared as to the amount that 

 they presumably have changed from the base line of their most 

 recent common ancestor. Thus crocodiles are more primitive than 

 birds. Situations are also known where primitive types have per- 

 sisted and specialized forms disappeared. That these can be "ex- 

 plained" as the persistence of less specialized groups against the 



