154 M. Agassiz on the Relations between Animals 



and when information derived from all possible quarters shall 

 have equally its due influence upon our natural methods. A 

 desire to investigate the various questions bearing upon classi- 

 fication has led me to revise the subject of the natural relations 

 which exist between animals and the elements in which they live. 

 The connection between animals and the surrounding media in 

 which they live has of late been so entirely disregarded, that it is 

 time to reconsider this question with all the attention its import- 

 ance demands, since we find in it a decided relation to the struc- 

 ture and functions of all animals. For though it is plain that 

 the mere living in water or upon dry land is in itself of slight 

 importance, as there are so many animals which dwell in the two 

 elements although having the same identical structure, it should 

 not be overlooked that the greater number of aquatic animals 

 have structural peculiarities common to all, and that the same is 

 the case with the terrestrial or aerial animals. For instance, all 

 those which live upon dry land breathe directly the atmospheric 

 air, and have a respiratory apparatus adapted for direct intro- 

 duction of this element into their systems, while aquatic animals 

 breathe through apparatus of a different structure adapted to a 

 permanent contact with aerated water. This circumstance alone 

 would suflBce to show that the natural relations of animals with 

 the elements in which they naturally dwell, is in direct connec- 

 tion with at least some of their structural peculiarities. But 

 there are other circumstances which may lead to the conviction 

 that this connection has not merely reference to the structure of 

 their respiratory apparatus, but influences their whole organi- 

 zation. The greater pressure under which aquatic animals are 

 maintained throughout their life modifies, in many other respects, 

 their organization. In many of them the surrounding element 

 has lai-gely a direct access into the cavities of the body or even 

 into their tissues ; so that a direct and universal influence of the 

 surrounding media must be acknowledged throughout the ani- 

 mal kingdom as soon as we take into consideration all their pe- 

 culiarities. This influence will be appreciated more correctly, if 

 we consider it separately m each great group of the animal king- 

 dom as established upon anatomical 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 connection which exists 

 between them and the element in which they live will not affect 

 at all the views which we shall entertain about that class, and 

 only allow us to consider within more natural limits, the true re- 

 lation which exists between fishes and the natural element in 

 which they are found. The circumstance that so many birds are 

 aquatic in their habits will no longer prevent us from considering 



