134 BIOLOGY AND ITS MAKERS 



point in natural history, and subsequent progress in system- 

 atic botany and zoology resulted from the application of the 

 methods of Cuvier and Von Baer, rather than from following 

 that of Linnaeus. His nomenclature remained a permanent 

 contribution of value, but the knowledge of the nature of 

 living forms has been advanced chiefly by studies in com- 

 parative anatomy and embryology, and, also, in the applica- 

 tion of experiments. 



The most significant advances in reference to the class- 

 ification of animals was to come as a result of the accept- 

 ance of the doctrine of organic evolution, subsequent to 

 1859. Then the relationships between animals were made 

 to depend upon community of descent, and a distinction 

 was drawn between superficial or apparent relationships 

 and those deep-seated characteristics that depend upon close 

 genetic affinities. 



Alterations by Von Siebold and Leuckart. — But, in the 

 mean time, naturalists were not long in discovering that the 

 primary divisions established by Cuvier were not well bal- 

 anced, and, indeed, that they were not natural divisions of 

 the animal kingdom. The group Radiata was the least 

 sharply defined, since Cuvier had included in it not only those 

 animals which exhibit a radial arrangement of parts, but also 

 unicellular organisms that were asymmetrical, and some of 

 the worms that showed bilateral symmetry. Accordingly, 

 Karl Th. von Siebold, in 1845, separated these animals and 

 redistributed them. For the simplest unicellular animals he 

 adopted the name Protozoa, which they still retain, and the 

 truly radiated forms, as starfish, sea-urchins, hydroid polyps, 

 coral animals, etc., were united in the group Zoophyta. Von 

 Siebold also changed Cuvier's branch, Articulata, separating 

 those forms as Crustacea, insects, spiders, and myriopods, 

 which have jointed appendages, into a natural group called 

 Arthropoda, and uniting the segmented worms with those 



