INTRODUCTION 7 



the carbon dioxide. This is particularly the case in animals, and 

 the term respiration is most frequently used to imply this exchange 

 of carbon dioxide for oxygen. The other waste products, corre- 

 sponding superficially to the ash of the fire, cannot be eliminated 

 in this way, and the process of removing them is termed excretion. 

 This getting rid of the substances resulting from the break down 

 of living matter, i.e. excretion, must be carefully distinguished 

 from the removal of the insoluble material taken in with the food, 

 but never at any time forming an actual part of the organism. 

 These breaking down processes are collectively termed Katabolisni. 

 The power to carry out the constructive and destructive changes, 

 together included in the term Metabolism, constitutes the second of 

 the fundamental vital phenomena, and we shall investigate this 

 matter more fully later after a fuller examination of the nature of 

 the living substance. 



Growth and Reproduction. 



The organism, then, is the seat of two antagonistic processes, 

 anabolism and katabolism. If the two are about balanced a con- 

 dition of relative stability is attained, whereas if the former is in 

 excess of the latter, then an increase in size or, in other words, 

 growth takes place. The growth is of a peculiar form, for the new 

 material is added in extremely minute particles, throughout the 

 whole of the living substance, a process termed growth by intussuscep- 

 tion. A crystal, on the other hand, even if it does increase in size, 

 does so by the deposition of layers upon its surface ; this is growth 

 by accretion. The former variety is definitely characteristic of 

 living things. 



In lower forms of animals, when they reach a certain size, 

 determined probably by physico-chemical requirements, they 

 divide into two separate daughter organisms, each of which, in the 

 presence of an adequate food supply, proceeds to grow until it 

 reaches a maximum size. Thus it will be seen that from the one 

 original animal two have been produced, in other words, Reproduc- 

 tion or Multiplication has taken place, and, further, it is clear that 

 in such cases we can regard it as discontinuous growth. All organisms 

 possess this power of reproducing their like, although it is not 

 always so clearly a case of discontinuous growth. In higher forms 

 two parent individuals are concerned, termed respectively the 

 male and the female, each of which produces as a part of itself a 

 tiny particle of living matter, the male or the female germ cell. 

 These two germs unite, a phenomenon known as fertilisation, and 

 from their union a single body results that will grow up like its 

 parents in all essential respects. Such a process is not met with 



