16 DOCTRINE OF EVOLUTION 



when our attention is directed to it, but it is something 

 which may have escaped our notice because it is so 

 natural and universal. The trunk of a tree bears the 

 limbs and branches and leaves above the ground, while 

 the roots run out into the surrounding soil from the 

 foot of the trunk; they do not grow up into the ah-. 

 An animal walks upon its legs, the wings of a bird are 

 just where they should be in order that they may be use- 

 ful as organs of flight. And these mechanical adjust- 

 ments in the case of living creatures occur for the same 

 reason as in mechanisms like the steamship, which has 

 the propeller at its hinder end and not elsewhere, and 

 which bears its masts erect instead of in any other way. 

 The next step in the analysis of organisms reveals the 

 same wonderful though familiar characteristics. The 

 living organism is composed of parts which are called 

 organs, and these differ from one another in structural 

 and functional respects. Each of them performs a 

 special task which the others do not, and each differen- 

 tiated organ does its part to make the whole creature an 

 efficient mechanism. The leg of the frog is an organ 

 of locomotion, the heart is a device for pumping blood, 

 the stomach accomplishes digestion, while the brain and 

 nerves keep the parts working in harmony and also 

 provide for the proper relation of the whole creature to 

 its environment. So rigidly are these organs special- 

 ized in structure and in function that they cannot re- 

 place one another, any more than the drive wheels of 

 the locomotive could replace the smokestack, or the 

 boiler be interchanged with either of these. All of the 

 organs are thus fitted or adjusted to a particular place 

 in the body where they may most efficiently perform 



