CHAPTER XIV 

 TAXONOMY OR CLASSIFICATION 



UNDERLYING all present systems of classification of 

 living organisms is the doctrine of evolution, that all 

 organisms are descended from other more or less dis- 

 similar organisms and that in the course of such descent 

 there is an inherent tendency to vary. Classification is 

 an attempt to group organisms in a manner that shall 

 represent, as nearly as our knowledge permits, actual 

 genetic relationships. 



192. Species. The size and limits of the proposed 

 groups are influenced by convenience. The unit of bio- 

 logical classification is the species, a term however which is 

 difficult to define. A species includes all those individuals 

 that show as much resemblance to each other as they 

 might be expected to show if they were all known to be 

 descended from a common and comparatively recent 

 ancestor. As the genetic history of the individuals is not 

 known, the grouping into species is an expression of the 

 opinion of the biologist, and his opinion is based upon 

 judgment and experience. It should be kept in mind that 

 a species is a taxonomic idea* and is not an entity the 

 existence of which can be proved. For this reason, 

 taxonomists often disagree as to the limits of species. 

 The more experience a botanist has had with plants, 

 especially with living plants in their native habitat, the 



*"The name itself is but the expression of a taxonomic idea." Greene, 

 "Landmarks of Botanical History," p. 122. 



(151) 



