THE LIVING WORLD 193 



of the most diverse elements and sometimes of but two or 

 even of only one. 



Again, many minerals, such as crystals,* are bounded 

 by plain surfaces and, with very few exceptions,f none are 

 bounded by curved lines and surfaces, while living 

 organisms are bounded by such lines and surfaces. 



Yet again, if a crystal be cut through, its internal 

 structure will be seen to be similar throughout. But if 

 the body of any living creature be divided, it will, at the 

 very least, be seen to consist of a variety of minute 

 distinct particles, called "granules," variously distributed 

 throughout its interior. 



All organisms consist either (as do the simplest, mostly 

 microscopic, plants and animals) of a single minute mass 

 of protoplasm, or of a few, or of many, or of an enormous 

 aggregation of such before-mentioned particles, each of 

 which is one of those bodies named a "cell" (Fig. 28, 

 p. 1 95). Cells may, or may not, be enclosed in an investing 

 coat or " cell-wall." Each cell generally contains within it 

 a denser, normally spheroidal, body known as the nucleus. 



Now protoplasm is a very unstable substance (as we 

 have seen many substances are whereof nitrogen J 

 is a component part), and it possesses active properties 

 which are not present in the non-living, or inorganic 

 world. In the latter, differences of temperature will 

 produce motion in the shape of " currents," as we have 

 seen with respect to masses of air and water. But in a 

 portion of protoplasm, an internal circulation of currents 

 in definite lines will establish itself from other causes. 



Inorganic bodies, as we have seen, will expand with 



* See ante, p. 143. 



t Spathic and hematite iron and dolomite are such exceptions. 

 % See ante> p. 139. See ante pp. 150-157. 



N 



