236 ELEMENTARY SCIENCE 



are chiefly composed of them, and the muscles of animals 

 (meat) furnish us our chief supply of them. They are also 

 present in vegetable foods, especially in peas and beans. 

 (Why especially in peas and beans?) 



Protoplasm. The proteins gradually merge into that 

 mysterious, semifluid substance which Huxley first called 

 the physical basis of life. We call it mysterious because 

 we know so little of its origin or of its nature. With its 

 behavior we are familiar. Its presence in all living things 

 is observable, and all the "manifestations of life" seem to 

 depend upon its activity. It responds in definite ways to 

 changes in the conditions which surround it, yet some of 

 its activities do not seem to be of the nature of responses; 

 they seem to be self-initiated; at least scientists have never 

 been able to determine the causes of such activities. So 

 here, you see, we have a substance which is a sort of "holy of 

 holies" of life, into which man with all his cunning has not 

 yet been able to penetrate. In it are the mysteries of life, 

 and whether it be in the lowliest plant or in the brain of 

 the wisest man, it has, for aught we know, the same phys- 

 ical nature. At least it seems to be composed of the same 

 elements, and no element has ever been found in it which 

 is different from those found in the non-living world. 

 When this substance comes to be known, if it ever is, the 

 secrets of life will be known as they have never been known 

 before. This is the substance which you have already 

 learned to call protoplasm. 



The Nutritive Cycle. You should study carefully the 

 diagram on page 237, and try thoroughly to understand its 

 significance. For nothing so completely indicates the 



