550 MANUAL OF ELEMENTARY ZOOLOGY 



It will be seen that there are two distinct ways in which 

 life is necessary for its own continued occur- 

 Zf h Life. ntlnulty rence - It: alone can provide the kind of body 

 in which it occurs, and such a body cannot live 

 unless the life of its parent be continued in it. Thus 

 all life is part of a single, self-continued process. This, 

 however, is no more than might be expected. If, as we 

 have seen to be the case, life requires substances which do 

 not arise in lifeless nature and involves processes which 

 lifeless things cannot carry out, it is not likely to arise 

 spontaneously amid lifeless surroundings. It may well be 

 that a process so peculiar required for its starting conditions 

 which existed in a former state of the earth, but cannot 

 now be reproduced. These conditions, whatever they may 

 have been, must have included the presence of a factor 

 that endowed the first living beings with some degree of 

 that purposiveness without which they could not have 

 survived. The circumstances which brought into being 

 protoplasm in a state of metabolism must also have brought 

 into being a rudimentary capacity for purposive reaction. 

 It is interesting to speculate whether this could arise 

 by a fortuitous arrangement of unorganised matter, or 

 must be regarded as the development of some latent 

 tendency in the lifeless universe. In any case purposive 

 reaction has evolved to greater complexity as living things 

 themselves have evolved, and that presumably by varia- 

 tion and selection, if we accept this as the method of 

 evolution of organisms. However that may be, the facts 

 that we have here considered give a new importance to 

 that ceaseless activity by which living beings maintain their 

 existence amid their lifeless surroundings. If that activity 

 failed or were overborne, life, so far as we can see, would 

 cease for ever. 



