38 



EVOLUTION AND ANIMAL LIFE 



We should give at least brief attention to what may be called 

 the primary, or necessary, conditions of life. We know that 

 fishes cannot live very long out of water and that birds cannot 

 live in water. These, however, are conditions which depend on 

 the special ecological habits of these two particular kinds of 

 animals. The necessity of a constant and sufficient supply of 

 oxygen is a necessity common to both. It is one of the primary 

 conditions of their life. All animals must have air. Similarly 



*/ 



both fishes and birds and all other animals must have food. 



This, then, is an- 

 other of the pri- 

 mary conditions of 

 animal life. 



If water be held 

 not to be included 

 in the general con- 

 ception of food, 

 then special men- 

 tion must be made 

 of the necessity of 

 water as one of 

 the primary condi- 

 tions of life. Proto- 

 plasm, the basis of life, is a fluid, although thick and viscous. 

 To be fluid its components must be dissolved or suspended in 

 water. In fact, all of the really living substance in an animal's 

 body contains water. This water, so necessary for the animal, 

 may be derived from the general food, all of which contains 

 water in greater or less quantity, or it may be taken apart from 

 the other food by drinking or by absorption through the skin. 



We know, too, that if the temperature is below a certain 

 minimum point or above a certain maximum, these points vary- 

 ing for different animals, death takes the place of life. It is 

 familiar knowledge that many animals can be frozen without 

 being killed. Insects and other small animals may lie frozen 

 through winter and resume active life again in the spring. An 

 experimenter kept certain fishes frozen in blocks of ice at a tem- 

 perature of -15 C. for some time and then gradually thawed 

 them out unhurt. There is no doubt that every part of the 

 body, all of the living substance, of these fish was frozen, for 

 specimens at this temperature could be broken and pounded up 



FIG. 28. The fiddler crab, Gehisimus. (Photograph by 

 Miss Mary Rathbun.) 



