THE LIVING ORGANISM 16 



pebble, a granite block, or a solid peak of the Rocky 

 Mountains. It is true that there is a considerable ran^e 

 in size between the microscopic bacterium at one ex- 

 treme and the elephant or whale at the other, but this 

 is far less extensive than in the case of lifeless things 

 like water and stone. In physical respects, water may 

 be a fluid, or a gas in the form of steam, or a solid, as a 

 crystal of snow or a block of ice. But the essential 

 materials of living things agree throughout the entire 

 range of plant and animal forms in having a jellylike 

 consistency. 



But by far the most striking and important character- 

 istic of living things is their definite and restricted 

 chemical composition. Out of the eighty and more 

 chemical elements known to science, the essential 

 substance of living creatures is formed by only six to 

 twelve. These are the simple and obvious character- 

 istics of living things which are denoted by the word 

 ^^organic." Everyone has a general idea of what this 

 expression signifies, but it is important to realize that 

 it means, in exact scientific terms, — constituted in 

 definite and peculiar ways. 



The living thing, then, possesses a definite constitu- 

 tion, which is a mechanical characteristic, while further- 

 more it is related to its surroundings hi a hard and 

 fast way. Just as locomotives are different in structure 

 so that they may operate successfully under dilTerent 

 conditions, so the definite characteristics of living things 

 are exactly what they should be in order that organ- 

 isms may be adjusted or fitted into the i^lares in nature 

 which they occupy. This universal relation to the en- 

 vironment is called adaptation. It is only too obvious 



