1906.] on The Phijsical Basis of Life. 399 



the protoplasm on which it operates — are both less in extent and 

 much more regular than are the movements of the vibrios. 



In the choice of food particles, and in the sorting of the coins, 

 there is an end to be served. Looked at from this point of view, 

 chemiotaxis sometimes presents novel features. Amceba proteiis, large 

 and slow-moving, frequently captures an active ciliate called Colpidium. 

 Observers describe the capture as being due to the Colpidium swimming 

 as though attracted into the pseudopodial jaws, whence it makes no 

 efforts to escape. Here the element of purpose, looked at from the 

 standpoint of Colpidium, is that of a Christmas ox marching to the 

 kitchen to be converted into beef-steaks. 



The directive effect of the medium upon a free cell is usually 

 more complex than in the case considered. Opalina is a large ciliate 

 which in a uniform medium swims straight fonvard, owing to the 

 movement of the cilia or vibratile hairs which cover its surface. The 

 movement in this case starts at the front end of the animal and sweeps 

 back as a wave like the wave over a cornfield. In a heterogeneous 

 medium the movement starts excentrically, the waves sweep obliquely 

 down the animal, and the direction of motion changes. The net 

 result again is the same — the animal ceases to be distributed evenly 

 when the water ceases to be of uniform composition. 



The next example raises the question of choice to a higher level. 

 It brings into the response of the animal its previous history. We 

 will take the simplest case, as it is offered by OpaVma. This animal 

 is parasitic in the intestine of the frog, and it thrives in a very slightly 

 acid medium. But its attraction to acid is not an inalienable quality. 

 Glut it with acid, soak it in dilute acid for an hour, and it now collects 

 in a region of alkali ; bathe it for an hour in very dilute alkali, and 

 its chemiotactic response is once more changed — it collects about the 

 acid. 



The mechanism underlying this change of response must be patent 

 to every chemist. There are many substances whose chemical and 

 physical characters are completely reversed by change from a trace 

 of acid to a trace of alkali, or vice versa. Amongst these sub- 

 stances, 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 imfiuence 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 contract 



