GEOGBAPHICAL DISTBIBUTION OF ANIMALS. 493 



tory apparatus. From this it results that this function is 

 then shackled, and that the animal may die asphyxiated in 

 the air, whilst he found in water that which he required to 

 breathe freely. To be convinced of the importance of these 

 variations in the physical condition of organs placed in air or 

 in water, it is sufficient to recal what takes place in our prac- 

 tical or dissecting-rooms. An anatomist desirous of study- 

 ing the structure of a delicate part, would attain his object 

 with difficulty, if he made his dissection with the part exposed 

 simply to the air ; but by placing under water the object of 

 his study, he is thereby enabled to distinguish much more 

 readily all its parts ; for these parts, supported in some mea- 

 sure by the liquid, preserve then their natural relations as if 

 they had a rigid and consistent tissue. Another circum- 

 stance which has an equal influence over the possibility of 

 life in air or in water, is the evaporation which always takes 

 place from the surface of the organized bodies when placed in 

 air, but which does not happen in water. A certain degree 

 of desiccation causes all organic tissues to lose their distin- 

 guishing physical properties, and we constantly observe that 

 losses by evaporation cause the death of animals when it goes 

 beyond certain limits. It results from this, that beings 

 whose organization is not calculated so as to preserve them 

 from the injurious effects of such an evaporation, can live- 

 only in water, and perish promptly in the air. Now the 

 animal economy can only meet this exigency by means of a 

 great complication in its structure. In fact, it the respira- 

 tion must be active, it becomes necessary that the respiratory 

 surface be then lodged profoundly in some internal cavity 

 where the air can only be renewed in the quantities necessary 

 for the support of life. To secure this renewal, it is essential 

 that the respiratory apparatus be complicated with motor 

 organs proper to secure it ; to prevent the desiccation of any 

 portion of the surface of the body, it becomes necessary also 

 that the distribution of the liquids in the various parts of the 

 body be accomplished easil}*, and that there exist an active 

 circulation, or otherwise that this surface be clothed with a 

 tunic scarcely permeable. This is so true, that even in fishes, 

 in which the circulation is so complete, but takes place 

 slowly, and in which the capillary network is not very close, 

 death takes place rapidly, as a necessary consequence of 

 the desiccation of a part of the body of the posterior por- 

 tion, for example even when this portion alone be exposed 



