536 ZOOLOGY. 



logically and anatomically, these aquatic and terrestrial 

 animals, we discover, at least in part, the causes of these 

 differences in their mode of existence. 



In studying respiration, we have pointed out a constant 

 relation between the intensity of this function and the vital 

 energy. Animals, we have said, consume in a given time an 

 amount or quantity of oxygen always the more considerable 

 that their movements are more lively and their nutrition more 

 rapid. Now, they can only obtain this oxygen in the fluids 

 with which their bodies are bathed, and in a litre of air 

 (1760773 pints) there exist 208 cubic centimetres (eighty 

 cubic inches, nearly) of this vivifying principle ; whilst in a 

 litre of water there exist dissolved merely about thirteen cen- 

 timetres (five cubic inches). It is evident, then, that the 

 degree of activity in the respiratory function, indispensable to 

 the exercise of the faculties peculiar to the superior animals, 

 ought to be much more easily attained in air than in water, 

 and that by reason of this difference^alone a stay or residence 

 in this latter fluid must be and is interdicted to all the more 

 elevated beings in the animal scale. It is readily compre- 

 hended, in fact, that an animal which, in order to live, requires 

 to appropriate to itself at every instant a considerable quan- 

 tity of oxygen, cannot find it in sufficient proportion when 

 plunged under water, and that then it must perish asphyxiated. 

 But, at first view, it seems less easy to explain the causes by 

 which an aquatic animal cannot continue to live when with- 

 drawn i'rom the water and placed in air, for it is then furnished 

 with a liquid richer in oxygen than was the liquid, the vivify- 

 ing action of which sufficed for all its wants. There are, how- 

 ever, various circumstances which, to a certain point, explain 

 chis phenomenon. Thus we learn by physics (natural philo- 

 sophy), that a body weighed successively in air and water, is 

 lighter in this latter than in the former, and that to maintain, 

 it in equilibrium, a weight equivalent to that which repre- 

 sented its weight in air, less that of the mass of water it has 

 displaced, is then sufficient. From this it results that ani- 

 mals whose tissues are too soft to support themselves in the 

 air, and which collapse to such a degree as to become unfit to 

 perform their functions in the organism, may yet live well in 

 the bosom of the waters where these same tissues, being 

 scarcely denser than the surrounding fluid, have occasion to 

 offer merely a feeble resistance to preserve their forms, and to 

 preserve the different parts of the body from collapsing on 



