THE LIVING ORGANISM 19 



all of these have a biological problem to solve which 

 comprises eight terms or parts, no more and no less. 

 This is surely an astonishing agreement when we con- 

 sider the varied forms of living creatures. And perhaps 

 when we see that this is true we may understand why 

 adaptation is a characteristic of all organisms, for they 

 all have similar biological problems to solve, and their 

 lives must necessarily be adjusted in somewhat similar 

 ways to their surroundings. k 



Carrying the analysis of organic structure one step 

 further, it is found that the various organisms are them- 

 selves complex, being composed of tissues. A frog's leg 

 as an organ of locomotion is composed of the protecting 

 skin on the outside, the muscles, blood vessels, and nerves 

 below, and in the center the bony supports of the whole 

 limb. Like the organs, these tissues are differentiated 

 structurally and functionally, and they also are so 

 placed and related as to exhibit the kind of mechanical 

 adjustment which we call adaptation. The tissues, 

 then, in their relations to the organs are like the organs 

 in their relations to the whole creature, i.e. adapted 

 to specific situations where they may most satisfactorily 

 perform their tasks. 



Finally, in the last analysis, all organisms and organs 

 and tissues can be resolved into elements which are 

 called cells. They are not little hollow cases, it is true, 

 although for historical reasons we employ a word that 

 implies such a condition. They are unitary masses of 

 living matter with a peculiar central body or nucleus, 

 and every tissue of every living thing is composed of 

 them. 



The cells of bone differ from those of cartilage mainly 



