THE ANIMAL IN 7 HE WORLD 543 



lifeless things processes comparable to those which are 

 carried out by the energy of living matter? This is, of 

 course, a separate problem in the case of each of the modes 

 in which the energy is exhibited. It is not possible to 

 discuss the question here, but we may say broadly that it 

 is as yet unanswered. For instance, the most conspicuous 

 case — that of movement — is due to the process known as 

 contraction, in which, as we have seen, a portion of living 

 substance changes in shape, though not in size, growing 

 shorter in one direction, but wider in others. The claim 

 is made that this process does not take place by internal 

 activity in lifeless matter, but it has been shown that 

 certain artificial emulsions (pp. 114, 115), when they are 

 divided into small drops and placed in water, will move by 

 their own activity, undergoing changes in shape which are 

 alleged (though this is disputed) to be of the same nature 

 as the contraction of living matter. In the same way 

 secretion and excretion have not been explained by any 

 process, such as filtration or osmosis, that takes place in 

 lifeless matter, and it has been claimed, but is disputed, 

 that such an explanation is impossible. On this point all 

 that can be said at present is that no complete explanation 

 of any of the processes in question has yet been given in 

 terms of chemistry or physics. 



4. The mere fact of the incorporation of new matter is not 

 peculiar to living beings. Any liquid will take up into 

 itself substances that are soluble in it. But we have seen 

 that assimilation is the manufacture by living substance, out 

 of unlike materials, of additional matter of its own com- 

 position. It has not yet been shown that this process is 

 paralleled in any kind of lifeless substance. It is true that 

 a suggestive analogy may be found in certain chemical re- 

 actions that take place with greater readiness in the presence 

 of a trace of one of their products, which may then be said 

 to assist in its own manufacture. In certain circumstances, 

 for instance, the action of nitric acid upon copper is 

 hastened by the presence of copper nitrate. 1 But in such 

 cases the presence of the substance helps rather than starts 



1 An instance of a reaction in which organic substances exhibit this 

 phenomenon of " autocatalysis " is the hydrolysis of certain esters, 

 which is aided by the presence of the free acid.. 



