102 GEOGRAPHICAL DISTRIBUTION 



investigation in water, he much more easily succeeds in distin- 

 guishing all the parts ; because these parts, sustained in a mea- 

 sure by this liquid, then preserve their natural relations just as if 

 they were of a consistent and stiffer tissue. Another circum- 

 stance which influences the possibility of living in air or in water 

 is the evaporation which always takes place from the surface of 

 organized bodies placed in the air, but which cannot take place 

 in water. A certain degree of dessication causes all organic 

 tissues to lose their distinguishing physical properties, and we find 

 that losses by evaporation always produce death in animals when 

 they exceed certain limits. It follows that creatures whose 

 organization is not calculated to preserve them against the injuri- 

 ous effects of evaporation, can only live in water and quickly 

 perish in air. Now the animal -economy is equal to this exigence 

 only when it possesses a very complicated structure. In fact, if 

 an active respiration be requisite, the respiratory surface must be 

 deeply lodged in some internal cavity where the air can be renew- 

 ed only in proportion as it is required for the support of life. To 

 secure this renovation, the respiratory apparatus must be furnish- 

 ed with proper motive organs ; to prevent the dessication or drying 

 of any portion of the surface of the body, the diffusion of the liquids 

 to the different parts of the body must be easily carried on, and 

 there must be an active circulation, or the surface must be in- 

 vested by a tunic or covering that is scarcely permeable. This 

 is so true, that even in fishes, yi which the circulation is very 

 complete, although slowly carried on, and the capillary net-work 

 not very dense, death speedily takes place in consequence of 

 dessication of a part of the body, of the posterior portion, for 

 example, even when this portion alone is exposed to the air, while 

 the rest of the animal remains under water. 



We may add, too, that in water, feeding may be effected with 

 less perfect instruments of prehension than in air, where the 

 transportation of the food required by the animal is more difficult. 

 In all its most essential relations, life is, in a manner, more easily 

 .maintained in the midst of the waters than on the surface of the 

 earth ; in the atmosphere it demands more perfect and more com- 

 plicated physiological instruments : the water is the natural ele- 

 ment of animals lowest in the zoological series ; and if the pro- 

 ductions of the creation have succeeded each other in the same 

 order as the transitory states through which every animal passes, 

 during the period of its development, we may conclude that ani- 

 mate creatures first appeared in the midst of the waters, a con- 

 clusion in accordance with the observations of geologists and the 

 text of the Scriptures. 



In this manner the physiologist can account for the division of 

 animals between the two geological elements of the globe, water 



