Animals and the Elements in which they Live. 225 



luded to these facts in other papers, but only recall here, the 

 f]^reat difference which exists in these connections 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 

 structure which is widely different from the plan of structure 

 prevailing in Mollusca, though they also are chiefly aquatic ; 

 and that even the terrestial 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 Crustacea and worms, 

 which are permanently aquatic types, presenting simply a 

 transformation of those peculiarities of structure which en- 

 able 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 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 ap- 

 paratus, 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 re- 

 spect Cetacea with common Articulata. They have a pul- 

 monary 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 permanently 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 difference in this 

 respect. For, though in their perfect state. Fishes are ex- 

 clusively 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 Mammals, in their im- 

 perfect condition, is developed to be a permanent organ of 

 respiration, while it is reduced and disappears in the higher 

 classes in proportion as the lungs acquire a greater develop- 

 ment. In Fishes, on the contrary, the homologue of the lung 

 remains functionally and organically in a rudimentary state, 



VOL. XLIX. NO. XCVIII.— OCTOBER 1850. P 



