rp ee 
and the Elements in which they live. 393 
which led earlier naturalists, generally, to the classification of 
animals according to the media in which they live, should have 
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~ 
_ nent, while we have found that structure is the ruling principle, 
ary 
and that natural connection with the element, is the second 
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 organs to perform the same 
functions. Isshall 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- 
tons between the different types. ; 
Among 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 pre- 
_Vailing in Mollusca, though they also are chiefly aquatic; and 
that even the terrestrial types of Mollusca present, for adaptation 
to an aérial mode of life, only a modification of their aquatic 
ypes. 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 peculi- 
arities of Structure which efiable the lowef classes to live under 
er, such as -will.enable theny to rise in their adult state into an 
wat 
aérial condition of e Among Vertebrata the case is very 
different. The type is coristructed for a terrestrial and aérial 
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- 
hently in the Sea; but, for all their structure, they are truly aérial 
animals. And this is equally the case with Birds and Reptiles ; 
and with the Fishes I am prepared to show that there is uo 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 aérial classes of Vertebrata. The difference here is 
Only this that the branchial apparatus, which exists simultane- 
ously in Reptiles, Birds, and Mammalia, in their imperfect condi- 
Srconp Sens, Vol. IX, No. 27.—May, 1850. 50 
