142 ^WE ANIMAL KINGDOM 



readily available, and it provides for economy in the length of descrip- 

 tions. Hiese are practical considerations, and were the motivating forces 

 behind the establishment of our present taxonomic system, which de- 

 veloped mostly during the 18th and 19th centuries. 



Since the middle of the 19th century, however, taxonomy has had 

 an additional and equally significant role. The grouping of animals is 

 used not only as a matter of convenience, but also in an attempt to 

 indicate the degree of evolutionary relationship present. Thus, the 

 species of one genus are considered to be more closely related to one 

 another than to the species of other genera, and to have evolved from 

 a single original species. Similarly, the genera of a family are considered 

 to form an evolutionary unit as well as a taxonomic unit, and so on. 

 Taxonomy can never indicate evolution exactly, since of necessity the 

 taxonomic boundaries between groups must be sharp whereas evolu- 

 tionary relationships form something closer to a continuum. Furthermore 

 taxonomy cannot describe the time dimension involved in any discussion 

 of evolutionary paths. Nonetheless the system has been revised continu- 

 ally to serve as well as possible as a framework from which evolutionary 

 relationships can be discussed. The analysis of evolutionary relationships 

 among organisms with taxonomy as the basic tool is the science of 

 systematics. 



Although the usages of genus and species are standardized by inter- 

 national rules and official lists of genera are maintained, the higher 

 taxonomic categories are less well regulated. There is no universal agree- 

 ment about either the number or the names of higher categories. Authors 

 have different opinions, depending upon their conclusions regarding 

 evolutionary relationship. The chordates, for example, are a group of 

 animals (including the vertebrates) having a notochord and other char- 

 acters in common, and are a basic group having the rank of a phylum. 

 In some organisms, e.g., the acorn worms, however, the existence of a 

 notochord is debatable. Students who believe it is absent place such 

 animals in a separate phylum (Hemichordata or Enteropneusta) from 

 the others (Chordata) while those who believe it is present arrange these 

 animals in one phylum (Chordata). Such differences of opinion persist, 

 and are not arbitrated by the International Commission. 



A partial classification of the animal kingdom is given in the Ap- 

 pendix. All phyla and most of the classes are included (arranged accord- 

 ing to the views of these authors). Many of the orders of common 

 animals are given, with a few examples of each. 



44. Definitions 



A discussion of taxonomy would be incomplete without definitions 

 of the different grouping levels. In a formal sense the species is defined 

 as a group of individuals capable of interbreeding under natural condi- 

 tions and reproductively isolated from other such groups. In practice 

 all of the necessary information is seldom available, and the species is 

 considered to be a group of individuals that could fit this definition and 

 which is recognizable as a distinct group by some dependable criterion 



