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) 
