and the Elements in wtdch they live. 155 



the class of Birds as a most natural group in the animal king- 

 dom, the limits of which are well defined by anatomical evidence ; 

 and the relations of aquatic birds to the waters upon which they 

 alight or in which they dive, will only be considered within the 

 limits of a well circumscribed natural group. The same may be 

 said of Reptiles ; and the circumstance that so many of their 

 types are almost entirely aquatic, while others are terrestrial, will 

 by no means prevent us from viewing them as a natural class, in 

 which the connection with either main land or the water shall 

 appear as a subordinate feature. 



Again, the class of Insects, which is so thoroughly aerial 

 throughout almost all its tj^pes, at least in their perfect state of 

 development, circumscribed as it is within natural limits upon 

 anatomical evidence, will appear to us as a type which shall bear 

 no relation in our mind to the class of Birds, although their 

 movement through the atmosphere be apparently so similar. 



But, although we remove in this manner almost completely 

 the circumstance of animals dwelling either in water or upon 

 main land as influencing in any way our general classification of 

 the animal kingdom, it were a great mistake 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 apparent, 

 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 subdivisions, and so much so, that I have arrived at 

 the conviction 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 consider- 

 ing those minor divisions let us take a general glance at the four 

 great types of the animal kingdom, beginning with the Radiata. 



If we consider the type of liadiata 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 natural types and the media in which they live. But if we 

 reduce the type of Radiata to those classes which 1 consider as 

 alone truly representing that type, we shall be at once struck with 



11* 



