12 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



Water. — Water cannot be withheld from living organisms for con- 

 tinued periods of time without injury and ultimate destruction. Proto- 

 plasm is full of water and suspends its functions when this water is 

 withdrawn. It is true that many animals are capable of existing with 

 extraordinarily little water and are extremely resistant to desiccation. 

 Meal worms (larvae of the beetle, Tenebrio molitor) have been known 

 to live in bran, dried at 105° C, for more than three weeks, and some 

 have survived the fourth week. The survivors of this experiment con- 

 tained 65% water, whereas the animals at the beginning of the experi- 

 ment contained only 61.5% water. 1 The assumption is that these 

 animals secure water by the chemical transformation of their food. 

 Such water is called water of metabolism. 2 ' 3 Tardigrades, rotifers, 

 nematodes, and other simple types of animals retain their vitality in 

 spite of long-continued drying in sunlight, and become active with 

 renewed water supply (Chapter XVIII). 



Sufficiency of water is most completely assured to animals which 

 live in it. The waters which collect on the earth's surface are rarely 

 even approximately pure; they usually contain, in solution, greater 

 or less amounts of the salts of sodium, calcium, magnesium, potas- 

 sium, and other substances. The solubility of inorganic substances in 

 water satisfies one of the primary requirements for the development 

 of life, and of plant life especially, though animals also obtain many 

 salts directly from the water. The larvae of Calcispongia and of sea 

 urchins are unable to develop a skeleton in water free from calcium. 

 Pure water is harmful to organisms. It acts by diffusing into their 

 cells and diluting the salt solutions of the protoplasm to an excessive 

 degree. 



Water containing salts in such concentration that no osmosis takes 

 place between it and the organism, i.e., water isotonic with proto- 

 plasm, is the most favorable medium for animal life. These conditions 

 are most closely approached by sea water. 4 The investigations of 

 L. Frederic 5 and others have shown that sea water is approximately 

 isotonic with the body fluids of the marine invertebrates, while this 

 is not true with respect to fresh water and its inhabitants. The ocean 

 accordingly affords its inhabitants the most favorable conditions of 

 life, a circumstance which has been explained by the supposition that 

 life originated in sea water. According to this theory, all living organ- 

 isms, both plant and animal, which occur elsewhere, must have 

 developed from marine ancestors and freed themselves from the 

 marine habitat in the course of geologic time. This is supported by 

 the fact that almost all the main branches of the animal kingdom 



