K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 183 



variations are more or less closely accumulated around a given 

 centre, according to the peculiar nature of each standard form ? 

 It follows at once from this that the transitions are more indi- 

 cated than realized, that they are very unequally distributed, 

 and that they never form an even progression ; for at the limits 

 of two spheres, the most similar members of each are far more 

 remote from one another than they are from their own centres. 



The conception of spheres does not exclude that of series ; on 

 the other hand, the different types, since a manifold degree of 

 development occurs in them, will necessarily arrange themselves 

 in series, which however again consist of spheres, radiating from 

 whose centres every single form varies in many directions. 



Four principal types appear distinctly to manifest themselves j 

 the type of the elongated, Articulated animal, the type of the 

 Radiate animal, the type of the Mollusk, and that of the Ver- 

 tebrate animal ; the latter unite the articulate and molluscous 

 type together in their animal and vegetative organs. Indeed 

 one might perhaps recognise in the head an outline of the ra- 

 diate type, since here in the course of further development all 

 parts become more and more collected around a central point. 

 Besides this, in any case the central portion of their nervous 

 system is something peculiar which is wanting in other animals ; 

 for which reason the Vertebrata cannot descend to the lowest 

 grades of organization. 



It will be seen that we have here arrived at the four principal 

 divisions of the animal kingdom established by Cuvier. We 

 believe, in fact, that Cuvier has penetrated most deeply into the 

 relations of animal organisms. But he does not satisfy us in 

 this ; that he requires in the Mollusca and Articulata not only 

 the type of their organization, but also a certain degree of deve- 

 lopment, a condition which can only be required of the single 

 classes. The consequence is, that all the animals of low organi- 

 zation are thrown among the Radiata, although very many of 

 them are by no means radiate in their structure. The bounda- 

 ries under these circumstances could only be drawn arbitrarily. 

 If Gordius belongs to the Articulata, why should not Filaria and 

 Vibrio ? It seems plain to us that the conditions which predo- 

 minate in this type may be traced down to the lowest stages of 

 organization ; only we must not require from these prototypes, 



