GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 537 



themselves. This single consideration suffices to explain 

 why gelatinous animals, such as the infusoria and medusae, 

 are necessarily confined to the waters ; for when we observe 

 one of these delicate beings still plunged in this liquid, we. 

 see that all its parts, even the most slender or delicate, sup- 

 port themselves in their normal position, and float with ease 

 in the surrounding medium ; but, so soon as we withdraw 

 them from it, their whole body collapses, and presents to the 

 eye merely a shapeless and confused mass. The influence of 

 the density of the surrounding medium on the mechanical 

 play of the instruments of life makes itself also felt on 

 animals whose structure is more perfect, but in which, how- 

 ever, respiration is performed by ramified membranous appen- 

 dages, like little brushes or bunches of feathers. Thus in the 

 aimelides, or even in fishes, the branchiae or gills are composed 

 of flexible filaments, which support themselves easily in the 

 midst of water, and in this way permit the respirable fluid to 

 reach, and to 'be renewed at all points of their surface ; but in 

 the air these same membranous filaments collapse by the 

 effect of their own weight, fall on each other, and by that 

 alone exclude the oxygen from the greater part of the respi- 

 ratory 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 



