THE EARTH THE ABODE OF LIFE 



volcanic or molten rocks of the earliest stages of the 

 earth's life. Think for a moment of what the simplest 

 forms of life are. A great deal has been heard of 

 microbes and bacteria during recent years, and we may 

 therefore assume that every one has some knowledge of 

 the structure of these simplest living things. They may 

 be compared to tiny bladders of jelly so small that the 

 microscope is necessary in order to see them, and some- 

 times so much smaller than this that the best micro- 

 scopes cannot distinguish them. Such forms of life are 

 called "unicellular organisms," because they consist of 

 a single cell, which contains the jelly-like substance called 

 protoplasm, and a smaller body, smaller even than these 

 tiny cells themselves, which is called the nucleus. 



These are the simplest forms of life. But all the higher 

 forms of life, and we may say, roughly speaking, any 

 form of life that the unaided eye can see, is made up not 

 of one cell but of many cells. A human being, for 

 example, is made up of uncounted millions of cells ; and 

 millions of cells go to the formation of a worm, a fish, 

 a gnat, or indeed to the formation of the simplest well- 

 known animal that the ordinary person could name. 

 Similarly millions of cells go to the formation of a leaf 

 or a twig. These higher forms of life are called " multi- 

 cellular organisms," because they have many cells, and 

 most often many different kinds of cells. For instance, 

 in the body of a man there are different kinds of 

 cells to form the skin, or the lining of the mouth, 

 or the substance of the eyes, or the red or white 

 corpuscles of the blood, or the grey matter of the 



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