INTRODUCTION 



9 



(i) 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 each 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; 



1 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 



