46 WHAT IS LIFE 



people are satisfied if they have named a pheno- 

 menon and others are quite happy if they have 

 restated the terms of the occurrence in fresh 

 phraseology, but it must be quite obvious that 

 neither of these processes helps one very far along 

 the road which leads to an explanation of the pro- 

 cess itself. 



Let us, however, suppose for a moment, that the 

 so-called rheotaxy is really a reflex action set up 

 by the running water. At any rate it falls into line 

 with a number of so-called other taxies, such as 

 photo-taxy or the turning of certain plants towards 

 the light and so on. 



These we must look upon as examples of the 

 power of irritability which the living protoplasm is 

 possessed of, a possession which Hertwig describes 

 as its most remarkable peculiarity. Moreover l he 

 says that irritability exhibits its specific phenomena 

 through the special structure of the irritable sub- 

 stance or, in other words, that irritability is a 

 fundamental property of living protoplasm, but 

 that it exhibits itself, according to its own specific 

 structure, under the influence of the external world 

 in specific energies and irritabilities. 



The extreme school of mechanists deny that 

 Does irrita- there is any such thing, strictly speaking, as irrita- 

 bility and with their views as formulated by one of 

 I 0p. cit., s. 136. 



