ASSUMED PHYSIOLOGICAL MECHANISMS 35 



effects. In all these cases we can trace the 

 chain of cause and effect, whereas in the physio 

 logical case we cannot. Let it be granted 

 that hope may be entertained of some day 

 tracing the physiological chain. It is never 

 theless clear that the existence of such 

 a hope, however confidently expressed, must 

 not be confused with evidence. 1 In recent 

 years it has become the custom to speak 

 of the * mechanisms ' of physiological activi 

 ties. Until these supposed mechanisms have \ 

 been actually discovered it would be better 

 not to use language which implies far 

 more than what our present knowledge 

 warrants. 



The failure to be able to trace, at present, 

 the causal chain between stimulus and 



1 It appears to me that a very striking instance of this con 

 fusion is furnished by the reasoning which runs through Pro 

 fessor Loeb's recently published essay on The Mechanistic 

 Conception of Life (Chicago, and Cambridge University Press, 

 1912). Thus after describing the observation that an un 

 fertilised ovum may be excited to development by various 

 means, such as placing the ovum for a short time in hypertonic 

 sea-water, or simply piercing its outer membrane with a needle, 

 Professor Loeb proceeds to draw the extraordinary conclusion . 

 ' that the process of the activation of the egg by the sperma 

 tozoon, which twelve years ago was shrouded in complete 

 darkness, is to-day practically reduced to a physico-chemical 

 explanation.' 



I 



