INTRODUCTION 9 



(1) Definite Size. The size of living organisms varies within 

 definite limits. The smallest animals known are microscopic 

 blood parasites; the largest living animals are the whales. The 

 difference is great but definite, and eacjj kind of animal has a 

 characteristic size. Non-living bodies, on the other hand, may 

 be of any size; for example, water may exist as a particle of 

 vapor or as an ocean. 



(2) Definite Form. If animals were not constant in form, we 

 would be unable to distinguish one from another. Non-living 

 bodies usually have no definite form, but may, like water in a 

 lake-bed, assume the shape temporarily forced upon them. 



(3) Definite Chemical Composition. The elements found in 

 living matter are all found in non-living bodies, but in living 

 matter certain elements are combined so as to produce a sub- 

 stance known as protoplasm. These elements are present in a 

 typical animal in the following proportions: 



Carbon 



Oxygen 

 Nitrogen 

 Hydrogen 

 Sulphur 



Phosphorus 



Chlorine 



Potassium 



Sodium 



Magnesium 



Calcium 



Iron 



99 per cent of weight; 



i per cent of weight. 



(4) Definite Organization. The protoplasm contained in 

 the bodies of animals is not continuous in most cases but is 

 divided up into small units called cells (p. 13, Fig. 2). A cell is 

 a small mass of protoplasm containing a nucleus. The bodies 

 of some animals are composed of only a single cell (PROTOZOA, 

 Chap. II), but all of the more highly organized animals are made 



