194 L. Agassiz on the Natural Relations between 



The recent progress in zoology, and of the various branches 

 of natural history connected with it, has, however, opened 

 the prospect of farther improvements, even upon the basis 

 on which our classification at present rests. For embryology 

 is already displaying its vast influence upon zoological ques- 

 tions, and the time is not far distant, when its share in the 

 natural arrangement of animals will be as large as that of 

 comparative anatomy itself, 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 classification has led me to 

 revise the subject of the natural relations which exist be- 

 tween animals, and the elements in whicli 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 importance demands, since we find in it a decided rela- 

 tion to the structure 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 pecu- 

 liarities common to all, and that the same is the case with 

 the terrestial or aerial animals. For instance,, all those 

 which live upon dry land, breathe directly the atmospheric 

 air, and have a respiratory apparatus adopted for direct in- 

 troduction 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 suffice to show that the natural 

 relations of animals with the elements in which they natur- 

 ally dwell, is in direct connection with at least some of their 

 structural peculiarities. Eut 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 organization. Th6 

 greater pressure under which aquatic animals are main- 

 tained throughout their life, modifies, in many other respects, 



