CUVIER AND BAER. 53 



The Radiate animals, finally, differ from tlie three other 

 principal forms by their body being the combination of fonr 

 or more main sections united in the form of radii (antimera). 



The distinction of these four principal forms of animals, 

 which has become extremely productive in the development 

 of zoology, is commonly ascribed entirely to Cuvier. How- 

 ever, the same thought was expressed almost simultaneously, 

 and independently of Cuvier, by Bar, one of the greatest 

 naturalists, and still living, who did the most eminent service 

 in the study of animal development. Bar showed that in the 

 development of animals, also, four different main forms (or 

 types) must be distinguished. ^^ These correspond with 

 the four plans of structure in animals, which Cuvier distin- 

 guished on the ground of comparative anatomy. Thus, for 

 example, the individual development of all Vertebrate ani- 

 mals agrees, from the commencement, so much in its funda- 

 mental features that the germs or embryos of different 

 Vertebrate animals (for example, of reptiles, birds, and 

 mammals) in their earlier stages cannot be distinguished at 

 all. It is only at a late stage of development that there 

 gradually appear the more marked differences of form which 

 separate those different classes and orders from one another. 

 The plan of structure, which shows itself in the individual 

 development of Articulate animals (insects, spiders, crabs), 

 is from the beginning essentially the same in all Articulate 

 animals, but different from that of all Vertebrate animals. 

 The same holds good, with certain limitations, in Molluscous 

 and Radiated animals. 



Neither Bar, who arrived at the distinction of the four 

 animal types or principal forms through the history of the 

 individual development (Embryology), nor Cuvier, who 

 4 



