THE PRINCIPLES OF TAXONOMY 143 



(usually morphologic). Conceptually the species is an evolutionary unit, 

 regardless of the method by which actual species have been sorted out. 



The genus is defined as a group of closely related species. This is 

 not as satisfactory a definition as the formal definition of a species, since 

 the word "closely" involves opinion. Actually, however, this does not 

 appear to be a serious problem. A survey of the genera in many different 

 taxonomic groups reveals that most taxonomists require about the same 

 degree of closeness for the species of one genus. 



The family is composed of related genera, the order of related 

 families, and so on. Since at each level the degree of closeness must be 

 evaluated, the definitions become less and less objective. It is apparent 

 from comparisons that what is an order in one phylum may be com- 

 parable with a class or a family in another. 



The classification system becomes more objective at the level of the 

 phylum. This level is reached more directly, of course, by dividing the 

 animal kingdom into a number of basic types. The phylum has been 

 defined as an assemblage of organisms showing some degree of relation- 

 ship among themselves and expressing as a whole a plan of existence that 

 is unique, fundamentally different from that of all other organisms. 

 Some people regard the phyla as unrelated, and therefore as objective 

 a category as the species. In practice, ho\sever, many of the phyla show 

 some similarity to one another, and a value judgment is still involved, 

 this time of distance rather than of closeness. Objectivity at the phyletic 

 level should be about the same as that at the generic level. 



45. The History of Taxonomy 



The development of our taxonomic system is one of the more excit- 

 ing chapters of biological history. Taxonomy was started by the Greeks 

 and Romans, most notably by Aristotle, but developed very little for 

 two thousand years, until the end of the 17th century. 



The first major break from this long era of stagnation is found in 

 the works of John Ray. Although zoology was only one of his several 

 interests, between 1676 and his death in 1705 he produced books on 

 birds, fishes, quadrupeds and insects. Ray introduced a more complex 

 grouping system than had been used before and improved greatly on 

 the language of description. He rejected entirely the whole mass of 

 superstition and medicinal folklore that had burdened earlier works. 

 Ray developed the key by which students can identify a given animal, 

 using only a few distinguishing characters. He also promoted the con- 

 cept of the genus as a group of closely similar species (without the added 

 concept of evolutionary relationship). 



The work of John Ray opened a new era. Many students of history 

 give him major credit for the development of a modern system of 

 taxonomy. It remained for another, however, to bring the new approach 

 into sharp focus and to initiate popular, world-wide activity in taxon- 

 omy. Linnaeus (Fig. 7.1) was the first taxonomist, in the sense that tax- 

 onomy was his career, his primary activity. Since Linnaeus made notable 

 contributions to the taxonomic system and since his work is enormous, 



