100 GEOGRAPHICAL DISTRIBUTION 



horns, spheres, cups, fans, shrubs, &c. ; some are studded with 

 fine stony needles ; others are sustained internally by flexible 

 fibres, arranged so as to form tubes and little cells. 



Common sponge, of which we make so much use, has a struc- 

 ture of the latter description ; it constitutes large brownish masses, 

 and is found in the Mediterranean. 



GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



To form a general idea of the animal kingdom, it is not enough 

 to know the principal phenomena by which life is manifest in 

 animate creatures, and 1o have studied the structure of their 

 bodies, and the mechanism of their functions ; we must also look 

 at the manner in which animals are distributed over the face of 

 the earth, and endeavour to appreciate the influence which the 

 different circumstances in which they are placed may exercise 

 over them. 



When we look at the manner of distribution of animals on 

 the globe, we are at first struck with the difference of the media 

 they inhabit. Some, as every body knows, always live under 

 water and quickly die when withdrawn from it ; others can only 

 exist in the air and almost immediately perish when submerged. 

 Some in fact are destined to inhabit the waters, and others to live 

 upon the land ; and when we compare aquatic and terrestrial 

 animals, in their physiological and anatomical relations, we find, 

 nt least in part, the causes of the differences in their mode of 

 existence. 



In studying respiration, we pointed out the constant relation 

 between the intensity of this function and vital energy. Animals 

 consume in a given time a quantity of oxygen, increasing in pro- 

 portion to the activity of their motions and rapidity of their 

 nutrition : now, they can obtain this oxygen only from the fluids 

 surrounding them ; in a gallon of air there are about 84 cubic 

 inches of this vivifying principle, while in a gallon of water we 

 ordinarily find only about five cubic inches. It is evident then 

 that the degree of activity in the respiratory function, indispen- 

 sable to the exercise of the faculties belonging to superior ani- 

 mals, must be of more easy attainment in air than in water, and 

 on account of this difference alone, the creatures highest in the 

 animal series cannot dwell in water. We comprehend, indeed, 

 that an animal which, in order to exist, must appropriate a consider- 

 able quantity of oxygen every instant, does not find it in suf- 



