1 86 SCIENCE PROGRESS 



chemical and physical characters are completely reversed by 

 change from a trace of acid to a trace of alkali, or vice versa. 

 Amongst these substances, and markedly possessed of this 

 character, are the chemical substances, called proteids, of which 

 all living matter is composed. The varied response to acid or 

 alkali may unquestionably be traced in the first instance to the 

 directive influence of the amphoteric proteid on the surface 

 energy of the animal and upon the train of chemical events in 

 its interior. 



A parallel differential response is furnished by Stentor — a 

 large, trumpet-shaped animalcule — which fixes itself by its foot 

 to some solid object. Touched on one side by a fine glass hair 

 very lightly, it bends towards the hair ; touched more heavily, 

 it bends away. Therefore there is a touch of a certain strength 

 which produces no response. To a series of touches regularly 

 repeated it gives the following responses. At first it simply 

 bends away, then it contracts right down on to its foot ; if that 

 does not get rid of the irritant, it looses its hold and swims away. 



These responses have been analysed with great care in order 

 to elucidate the underlying mechanism. I can stop only to point 

 out the element of purpose. In order to get rid of an irritation, 

 a certain movement is tried ; it fails, another movement is 

 tried ; it fails, and a third movement is tried. 



Now, it must be clearly understood that organisation will 

 account for these phenomena. Quite as remarkable a series of 

 responses, each in turn designed to get rid of an irritant, can be 

 obtained from a Frog which has been deprived of its brain, and 

 therefore presumably lacks both consciousness and intelligence. 

 And step by step, as organisation advances, the response gains 

 in complexity, until the human imagination is unable to unravel 

 the chain of cause and effect. But the biologist is cognisant of 

 no break in the series from the choice of a Vibrio, which can be 

 analysed algebraically, to the choice of a child between two toys. 



The faculty, clearly seen in the case of Stentor, of storing 

 impressions, so that the response to any particular stimulus is 

 in part conditioned by the stimuli which have preceded it, is 

 a familiar property of living matter, and also of matter in the 

 colloidal state. The molecular state of a jelly is not fixed by the 

 conditions of the moment. Just as a piece of wrought iron has 

 properties different from those of cast iron, so the circumstances 

 which attend the making of a jelly — temperature, concentration, 



