34 



GENERAL ZOOLOGY 



no two are alike, yet each species when adult has a certain 

 average size and form which enables one to recognize it. 

 A cat is never as large as a cow; a birch tree is never found 

 with the bark or leaves of an oak. Living things cling to 

 such specificities generation after generation. 



2. Precise Elementary Composition. Living matter, 

 though highly variable, is made up of only twelve of the 

 eighty odd chemical elements available in nature. These 



Excretions 



Oxtjqen 



Oxidation 

 O\erqy 



Rxeccs food material 



FIG. 20. A scheme showing metabolism, or the chemical changes continu- 

 ally going on in protoplasm. No living thing can stand still. It must change 

 continually. 



occur in definite proportions and form only certain kinds 

 of compounds. 



3. Definite Organization. Living organisms are composed 

 of a variety of unlike parts which are generally incapable 

 of independent existence. The smallest unit of organic 

 structure is a cell and in large animals there may be billions. 

 The parts of an animal will not exist separately you can- 

 not remove a kidney or a stomach and have it remain 



