226 OUTLINES OF EVOLUTIONARY BIOLOGY 



Genera in turn are grouped in families, separated from one 

 another by still more fundamental characters, and with sub- 

 families as intermediate groups where necessary. Families are 

 grouped in suborders and orders, orders in subclasses and classes, 

 classes in phyla (or subkingdoms), and phyla in kingdoms. Of 

 kingdoms only two are recognized, animals and plants, and even 

 these cannot be sharply separated from one another, because 

 amongst the lowest forms of life (Protista) the distinction between 

 animals and plants, as we have already seen, ceases to exist. 



The arrangement of individual organisms in species, and the 

 grouping of these to form genera, families, orders, classes, phyla 

 and kingdoms, constitutes the work of the systematist and is 

 usually spoken of as classification, while the study of the 

 principles in accordance with which classification should be 

 carried out forms a special branch of biological science sometimes 

 known as taxonomy. 



The first and greatest of the modern systematists was Karl von 

 Linne, now usually known as Linnaeus, a Swedish naturalist who 

 was born in 1707 and died in 1778. In his great work, the 

 " Systema Naturae," which is now recognized as the starting point 

 of modern systematic zoology and botany, he described all the 

 species of plants and animals then known, and endeavoured to 

 arrange them in a " natural system " in accordance with their 

 mutual resemblances and differences. As an aid to the 

 accomplishment of this formidable task he invented the binomial 

 system of nomenclature, in accordance with which each kind of 

 plant or animal is referred to by the name of the genus as well as 

 by that of the species to which it belongs. The necessity for the 

 constant repetition of longer or shorter descriptions as a means 

 of identification every time there is occasion to refer to a 

 particular species is thus avoided, while at the same time the use 

 of the generic name makes it possible to employ the same 

 specific name in a number of different genera without risk of 

 confusion, a very important point in consideration of the 

 enormous number of species which have to be distinguished from 

 one another. 



The binomial system of nomenclature has proved so well 

 adapted to its purpose that it has survived practically unchanged 

 to the present day, although the number of known species has 

 increased enormously in the interval and the Linnsean system 

 of classification has undergone profound modification at the hands 



