INTRODUCTION n 



protoplasm of a complex animal is not homogeneous, but is built 

 up of an enormous number of tiny vital units, just as a building 

 may be composed of bricks. Closer examination shows each unit 

 to consist of a certain amount of protoplasm containing within it a 

 denser portion. These tiny masses we call cells, a somewhat 

 misleading name. A cell then is a small portion of protoplasm 

 surrounding a denser and more opaque nodule, the nucleus. 



As we pass down in the animal scale we find that these cells 

 become relatively fewer in each individual, until finally, when we 

 come to the lowest forms, the whole creature is composed of just 

 one cell. This single cell, however, is capable of exhibiting all the 

 vital phenomena. We therefore find that from this point of view 

 we may divide animals into two distinct groups : one in which the 

 individual is composed of only one cell, these we term the Protozoa 

 or unicellular animals, and the other, in which the animal is built 

 up of a large number of cells often differing enormously among 

 themselves in size and shape, and these we designate the Metazoa 

 or multicellular animals. 



Although we have not actually defined life we have 

 enumerated certain of the functional and structural characters that 

 mark off the organic from the inorganic world, and this in itself 

 enables us to get a clearer idea of what is implied when we say that 

 a certain thing is alive. 



The world of living beings, of which we ourselves form an 

 integral part, presents an infinite number of problems to the enquiring 

 mind. So diverse and so complex are they that no one person can 

 ever hope to master more than a small group of them. We have 

 indicated above the various ways of approaching these questions, 

 and also endeavoured to set out those fundamental attributes of 

 organisms, more particularly animals, that are more or less common 

 knowledge, and often taken for granted without further thought. 

 The purpose of the succeeding chapters is to elaborate these two 

 points in a more thorough and systematic way, by the examination 

 of the main problems and the methods of treating them. To do 

 this we shall consider in a certain amount of detail a selected number 

 of animals differing both in structure and mode of life, and chosen 

 with a view to emphasising the course of evolution in the animal 

 kingdom. 



