8 AN INTRODUCTION TO ZOOLOGY 



in the inorganic world, so that in this power of being able to reproduce 

 their like living things differ from the non-living, and so it constitutes 

 a further vital characteristic. 



It has just been noted that in higher forms the union of 

 a male and female germ cell gives rise to a new individual which 

 undergoes a series of changes, often quite complicated, whereby it 

 grows more and more like its parent. This process we term develop- 

 ment, and it is usually accompanied by a marked increase in size 

 or growth. It is then a period when anabolism is in excess of 

 katabolism. 'At the conclusion of this period the animal is full 

 grown, and the two processes approximately balance. Somewhere 

 about the same time the animal becomes adult, that is to say, it 

 is able in its turn to produce germ cells, and so in conjunction with 

 another animal of the opposite sex give rise to number of new 

 individuals. After a longer or shorter period of adult life the 

 katabolic processes exceed the anabolic, with the result that, even 

 if no accident or illness supervenes, there finally comes a time when 

 some part of the organism that is of vital importance gives way, 

 and the animal ceases to live, or as we say, dies. Death is the normal 

 termination of this series of events, just as much as birth is its 

 beginning. We see, then, that an animal goes through a succession 

 of changes, beginning in the higher forms at fertilisation, including 

 growth, and ending with death, and somewhere between the middle 

 and the end possesses the power of reproduction. All these pheno- 

 mena together constitute an ordered whole termed the life history 

 or life cycle of the animal, and in exhibiting this organisms differ 

 from non-living things. We have now considered the main pheno- 

 mena characteristic of life, and seen that they are expressed as the 

 functions of living beings. They are in the main four : (i) Irrita- 

 bility ; (2) Metabolism ; (3) and (4) Growth and Reproduction, 

 which are closely linked and imply a life cycle. It is now necessary 

 to examine organisms from the point of view of their structure. 



Protoplasm. 



The above considerations have led us to see that there 

 are a number of points in which the behaviour of living and non- 

 living things differ, and the powers possessed by the former are not 

 shared by the latter. In considering the structure, too, we shall 

 find a striking and absolute difference. When we press our enquiries 

 to the limit we shall find that all organisms or, at any rate, the living 

 portions of them, are composed of a highly complex substance that 

 is termed Protoplasm, that is never found in the inorganic world. 

 This material is found wherever we find evidences of life, and it is 

 remarkably similar in chemical and physical properties in all 



