Animals and the Elements in which they Live. 195 



their organization. In many of them the surrounding ele- 

 ment has largely a direct access into the cavities of the body, 

 or even into their tissues ; so that a direct and universal in- 

 fluence of the surrounding media must be acknowledged 

 throughout the animal kingdom as soon as vjre take into con- 

 sideration all their peculiarities. This influence will be ap- 

 preciated more correctly, if we consider it separately in each 

 great group of the animal kingdom as established upon ana- 

 tomical evidence. 



After removing the whales from the fishes, it will be 

 plain that the Cetacea must be considered simply as an 

 aquatic type of the class of Mammalia, and that the connec- 

 tion which exists between them and the element in which they 

 live, will not aff'ect at all the views which we shall entertain 

 about that class, and only allow us to consider within more 

 natural limits, the true relation which exists between fishes 

 and the natural element in which they are found. The cir- 

 cumstance that so many birds are aquatic in their habits, 

 will no longer prevent us from considering the class of birds 

 as a most natural group in the animal kingdom, the limits of 

 which are well defined by anatomical evidence ; and the re- 

 lations 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 types, 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, al- 

 though their movement through the atmosphere be ap- 

 parently so similar. 



But, although we remove in this manner almost com- 

 pletely the circumstance of animals dwelling either in water 

 or upon mainland as influencing in any way our general 

 classification of the animal kingdom, it were a great mistake 



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