196 L. Agassiz on the Natural lielations between 



to lose sight entirely of this most intimate relation among 

 the natural secondary groups of animals under their different 

 types. 



The value of these considerations has become more ap- 

 parent since the outlines of the leading divisions in the 

 animal kingdom have been made in detail, by allowing the 

 results of embryology to have their due share of influence 

 upon our classification ; and the object of these remarks is 

 chiefly to show that there is a universal relation throughout 

 the animal kingdom between their structure and gradation 

 and the elements in which they live ; that in all the four 

 great types of the animal kingdom, the aquatic groups stand, 

 in natural classification, lower than the terrestrial, and that 

 this connection is so intimate as to extend even to the sub- 

 divisions, and so much so, that I have arrived at the convic- 

 tion that in an otherwise well-defined natural division, the 

 aquatic tribes should be placed below the terrestrial ones ; 

 that, even in narrowly circumscribed families the aquatic 

 genera rank below the terrestrial, and that even in natural 

 genera the aquatic species are inferior to the terrestrial 

 ones. But, before considering those minor divisions, let us 

 take a general glance at the four great types of the animal 

 kingdom, beginning with the Radiata. 



1. General View of the Radiata. — If we consider the type 

 of Radiata as it is still circumscribed in some of our most 

 recent works upon the animal kingdom in general, we may 

 fail to discover this intimate connection between their na- 

 tural types and the media in w;hich they live. But, if we 

 reduce the type of Radiata to those classes which I con- 

 sider as alone truly representing that type, we shall be 

 at once struck with the remarkable result, that all these 

 animals are aquatic, nay, that, with one single exception, 

 they are all marine. But, before this can be acknowledged, 

 it must be shown that the type of Radiata should be re- 

 duced to the three classes of Polypi, Jelly-fishes, (Medusa) 

 and Echinoderms ; and that, among Polypi, there are large 

 numbers of animals now united which do not all belong to 

 that class. The most extensive range acknowledged by 

 some zoologists in the type of Radiata includes Infusoria 

 with the Rotifera. and also intestinal worms. Without 



