491 



OF THE GEOGRAPHICAL DISTRIBUTION 

 OF ANIMALS. 



624. To form a general idea of the animal kingdom, it 

 is not sufficient merely to know the principal phenomena by 

 which life manifests itself in animated beings, and to have 

 studied the structure of their bodies and the mechanism of 

 their functions ; it is also necessary to take a comprehensive 

 and general view of the manner in which animals are spread 

 over the surface of the globe, and to endeavour to appreciate 

 the influence exercised, or which may be exercised, over them 

 by the various circumstances in the midst of which they are 

 destined to live. 



625. When we direct our attention to the manner in 

 which animals are distributed around us on the globe, w6 are 

 at first struck with the difference of the media in which they 

 live. Some, as every one knows, live always under the 

 waters, and die speedily when they are removed from this 

 liquid ; others can live only in air, and perish so soon as they 

 are immerged. Some, in fact, are destined to people the 

 waters, others to live on land ; and when we compare, physio- 

 logically and anatomically, these aquatic and terrestrial 

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

 ferences 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 centimetres 

 (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 comprehended, in 

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



