178 M. Agassiz on the Relations between Animals 



been so completely abandoned, and even considered of no value 

 at all in systematic classification. For my own part I have no 

 doubt that this negative result has arisen from the circumstance 

 that all aquatic animals were brought together, in these earlier 

 attempts, without reference to their structure or organic develop- 

 ment, while we have found that structure is the ruling principle, 

 and that natural connection with the element is the secondary 

 motive by which these connections are influenced. Indeed, 

 aquatic animals, though agreeing in many respects, and though 

 provided with analogous apparatus to perform the same functions, 

 have, in different types of the animal kingdom, a very different 

 plan of structure, and very different oi'gans to perform the same 

 functions. I shall not enter into a detailed illustration of these 

 differences, as I have alluded to these facts in other papers, but 

 only recall here the great difference which exists in these connec- 

 tions between the different types. 



Among the Radiata, which are all aquatic, we find even that the 

 adaptation to the liquid element is introduced in a plan of struc- 

 ture which is widely different from the plan of structure prevail- 

 ing in the Mollusca, though they also are chiefly aquatic ; and 

 that even the terrestrial types of Mollusca present, for adaptation 

 to an aerial mode of life, only a modification of their aquatic 

 types. The same may be said of Insects, in which the structure is 

 mainly that of the Crustacea and Worms, which are permanently 

 aquatic types, presenting simply a transformation of those pecu- 

 liarities of structure which enable the lower classes to live under 

 water, such as will enable them to rise in their adult state into an 

 aerial condition of existence. Among the Vertebrata the case is 

 very different. The type is constructed for a terrestrial and aerial 

 mode of life ; even their aquatic representatives have rudiments 

 of the apparatus, which acquire the highest development in the 

 complete terrestrial types, and most of their aquatic types are truly 

 aerial animals living in water, just as Insects are aquatic types 

 adapted to the air. Let us only contrast, in this respect, Cetacea 

 with common Articulata. They have a pulmonary mode of life 

 as much as man ; they have the same mode of reproduction ; only 

 their form enables them to dive under water and to dwell perma- 

 nently in the sea ; but, for all their structure, they are truly aerial 

 animals. And this is equally the case with Birds and Reptiles ; 

 and with the Fishes I am prepared to show that there is no differ- 

 ence in this respect. For though, in their perfect state. Fishes are 

 exclusively aquatic, they are completely built upon the same plan 

 with those aerial classes of Vertebrata. The difference here is 

 only this, that the branchial apparatus, which exists simultaneously 

 in Reptiles, Birds and Mammalia, in their imperfect condition, is 

 developed to be a permanent organ of respiration, while it is re- 



