250 ESSAY ON CLASSIFICATION 



this we see four rows of nervous threads along the spinal marrow, 

 which itself contains four strings, and a quadripartite grey mass. The 

 muscles of the trunk form also four principal masses, which are par- 

 ticularly distinct in the Fishes. The animal system is therefore dou- 

 bly symmetrical in its arrangement. It might easily be shown how the 

 vegetative systems of the body correspond to the type of Mollusks, 

 though influenced by the animal system. 



From the illustrations accompanying this discussion of the great 

 types or branches of the animal kingdom, and still more from the 

 paper published by K. E. von Baer in the Nova Acta/^ it is evident 

 that he perceived more clearly and earlier than any other naturalist 

 the true relations of the lowest animals to their respective branches. 

 He includes neither Bryozoa nor Intestinal Worms among Radiata, 

 as Cuvier and after him so many modern writers did, but correctly 

 refers the former to the Mollusks and the latter to the Articulates. 



Comparing these four types with the embryonic development, von 

 Baer shows that there is only a general similarity between the lower 

 animals and the embryonic stages of the higher ones, arising mainly 

 from the absence of differentiation in the body, and not from a typi- 

 cal resemblance. The embryo does not pass from one type to the 

 other; on the contrary, the type of eacfi animal is defined from the 

 beginning and controls the whole development. The embryo of the 

 Vertebrate is a Vertebrate from the beginning and does not exhibit 

 at any time a correspondence with the Invertebrates. The embryos 

 of Vertebrates do not pass in their development through other per- 

 manent types of animals. The fundamental type is first developed, 

 afterwards more and more subordinate characters appear. From a 

 more general type the more special is manifested, and the more two 

 forms of animals differ, the earlier must their development be traced 

 back to discern an agreement between them. It is barely possible 

 that in their first beginning all animals are alike and present only 

 hollow spheres, but the individual development of the higher ani- 

 mals certainly does not pass through the permanent forms of lower 



^ "Beitriige zur Kenntniss . . . ," Nova Acta .... XIII (1827), containing seven 

 papers, upon Aspidogaster, Distoma, Cercaria, Nitzchia, Polystoma, Planaria, and the 

 general affinities of all animals. These "Beitrage" and the papers in which Cuvier 

 characterized for the first time the four great types of the animal kingdom {Annales 

 du museun . . . , XIX, 1812), are among the most important contributions to general 

 Zoology ever published. 



