Life, its thysical basts and simplest expression 41 



the other hand if an aninlal which hves normaUy on the surface 

 of the earth is taken up a very high mountain or is carried up in 

 a balloon to a great altitude where the pressure of the atmos- 

 phere is much less than at the earth's surface, serious conse- 

 quences may ensue, and if too high an altitude is reached, death 

 occurs. 



Some animals require certain organic salts or compounds 

 of lime to form bones or shells, etc. These salts may be re- 

 garded as necessary articles of nutrition, though their function 

 is not that of ordinary food. These are peculiar demands of 

 special kinds of animals. There might also be included an:iong 

 primar}^ life conditions such necessities as the light and lieat of 

 the sun, the action of gravitation, and other jDhysical conditions 



Fig. 31. — Long-horned boring beetle, Ergates sp. — larva, pupa and adult insect. 



without which existence of life of any kind would be impossible 

 on this earth. 



Finally we may refer briefly to tlio "grand prol)lem " of tlic 

 origin of life itself. Any treatnirnt of this question is bound to 

 be wholly theoretical. We do not know a single positive thing 

 about it. We have some negative evidence. Tliat is, we liave 



