10 THE SCOPE OF ZOOLOGY 



subdivisions. Of course, there are many cases where these divisions 

 merge, as in the study of biochemistry or biophysics. 



The Nature of Life 



Since our primary concern in this book will be with one of the 

 biological sciences, it will be well to learn just what is meant by living 

 matter. Offhand, you might think that there is little need for such a 

 survey because the distinction may seem very simple. A cow grazing 

 in the field is very definitely alive, and a rock on a mountain side is 

 very definitely not alive. In tabulating some of the points of difference, 

 however, we can better understand the nature of living matter. 



1. Growth. Living matter has the unique distinction of growth. 

 Whether it be a tree, a worm, or a man — all living things take food 

 and convert it into a part of their bodies with a consequent increase in 

 size. All forms of life start their existence as small, usually micro- 

 scopic, bits of matter ; yet by the process of growth through food con- 

 version many of them are able to reach a very large size. Nonliving 

 matter has no growth in a comparable manner. 



There are some cases where nonliving matter appears to grow, 

 but it is in an entirely different manner. When you visit a large under- 

 ground cave you may see beautiful stalactites hanging from above and 

 stalagmites coming up from below. These become larger with the 

 passage of time by a special kind of growth. Water seeping down 

 from above contains minerals in solution, and as this water runs down 

 these formations some of the minerals are deposited to make the forma- 

 tions wider and longer. This method of increase in size is known as 

 accretion and involves the addition of material to the surface. There 

 are quite a number of examples of growth through accretion among 

 nonliving material. You can readily understand, however, that this is 

 entirely different from the growth of living organisms in which ma- 

 terials are incorporated from within. 



2. Cellular Organization. Cells are small units of life which form 

 the basis for the organization of most forms of living matter. Some 

 forms are composed of single cells; and still others, such as viruses, 

 do not have the complete cellular organization. However, with these 

 exceptions, we can say that all living matter has a cellular organization. 



3. Cell Respiration. All of the food which an animal takes into its 

 body or which a plant manufactures does not go to increase the size 

 of the body. Some of it is combined with oxygen for a consequent 

 release of energy. This is known as cell respiration and through it 

 an animal gets the energy necessary for its activities. It is possible to 



