134 BIOLOGY AND ITS MAKERS 
point in natural history, and subsequent progress in system- 
atic botany and zodlogy resulted from the application of the 
methods of Cuvier and Von Baer, rather than from following 
that of Linneus. 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 Zodphyta. 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 
