A DEFINITION OF EVOLUTION 



kind differing from each other only in minor traits, except sex; sharply 

 separated in some traits from all other species; and mutually fertile, but 

 at least partially sterile when crossed to other species. In the viewpoint of 

 Linnaeus, such species were absolute. "There are just so many species as 

 in the beginning the Infinite Being created." But many present-day biolo- 

 gists feel that the species is to some extent artificial, that is, the boundaries 

 between closely related species are arbitrary rather than natural. 



Taxonomic Categories. Linnaeus recognized that several species may 

 have so much in common that they must be grouped together as a genus, 

 distinct from other species-clusters ( genera ) . Thus, the current edition of 

 the American Ornithologists' Union Checklist of North American Birds 

 lists two species of Melanerpes and three of Agehius. But genera can be 

 much larger. Thus, the Checklist gives twelve species of the genus Puffimis 

 (shearwaters). And there are a few very large genera, for example the 

 weed Crepis, of which Babcock recognizes 196 species! 



The genera, also, Linnaeus found to fall naturally into larger clusters, 

 based on similarity in rather fundamental characters. These groups of 

 like genera he called orders. The genus Melanerpes belongs to the order 

 Piciformes, along with ten other genera of woodpeckers and allied birds. 

 Agehius belongs to the great order Passeriformes, including the great 

 majority of song birds. Finally, these orders were grouped into classes, the 

 diverse members of which share only very fundamental characteristics. 

 Thus, all of the orders of birds comprise the single class Aves. 



Linnaeus grouped the classes into the two kingdoms, Plantae and Ani- 

 malia, but did not feel the need of any category intermediate between 

 class and kingdom. Ernst Haeckel, in the immediate post-Darwinian era, 

 introduced the term pJujIum to include related classes. Phylum means, 

 etymologically, a line of descent, and the term was chosen especially for 

 its appropriateness to the new study of evolution. For a similar reason, 

 Haeckel introduced the faniihf as a category intermediate between genus 

 and order. 



Thus the full hierarchy of essential taxonomic categories is species, 

 genus, family, order, class, phylum, and kingdom. Everv organism which 

 is described must be assigned to each of these categories, either ONcrth- 

 or implicitly. Returning to the two birds discussed above, their complete 

 classification is as follows: 



Kingdom Animalia Animalia 



Phylum Chordata Chordata 



Class Aves Aves 



Order Piciformes Passeriformes 



Family Picidac Icteric! le 



Genus Melanerpes Ageluius 



Species en/llir()eej)haliis phoeniceus 



All oi the taxonomic categories are conimonh subdivided for purposes 

 of detailed study. The prefixes sub- and super- may be added to any of 

 the standard categories to indicate subdivisions or larger assemblages. 

 Other intermediate categories are sometimes used, but do not have official 



28 



