STRUCTURE OF ORGANISMS. 21 



last the others. And so we may imagine did the nicely 

 balanced mixture of anabolic and katabolic proteins 

 finally become elaborated into protoplasm. Innumer- 

 able compounds must, of course, have failed to establish 

 themselves in this way owing to too great fixity or too 

 great instability. For many reasons it seems probable 

 that life originated in the sea ; protoplasm contains the 

 same elements as sea-water, and in much the same 

 proportions. 



Speculating further, we may suppose that it became an 

 advantage for some of this vaguely defined metabolising 

 substance to become separated off into individual masses, 

 which came to acquire the structure of cells. From this 

 point onwards we can appeal to known evidence for our 

 history of the evolution of life. 



CHAPTER II 



THE CELLULAR STRUCTURE OF ORGANISMS : 

 REPRODUCTION AND DEATH 



ALTHOUGH all living organisms necessarily contain some 

 protoplasm, yet they are far from being entirely made up 

 of it. All the parts of an organism are not truly alive, 

 but only that portion which is protoplasmic. However, 

 the substances of which it is composed are either about 

 to be assimilated into protoplasm, or are the products of 

 protoplasm. Indeed the great bulk of a plant or animal 

 may be formed of the accumulated products of its past 

 activity. Such, for instance, is the woody supporting 

 tissue of a tree, the skeleton of a coral, the shell of a 

 snail, the bony substance or hair in. ourselves. While 

 the living element is continually undergoing change, the 

 dead deposit may continue unaltered during the life of 

 the organism, and even afterwards. 



A microscopic examination of living organisms teaches 



