THE PHYSICAL BASIS OF LIFE 185 



of particles are presented to Amoeba, and it sorts them very 

 imperfectly into those which modify the incessant streaming of 

 its protoplasm so that they become engulphed, and those which 

 do not so modify the streaming. The element of choice, or, as 

 we may now put it, the directive influence of the surroundings, 

 is much less perfect here than in the case of the Vibrios, 

 because the oscillations — the movements of 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. Amoeba 

 proteus, 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. Opaliua 

 is a large ciliate which in a uniform medium swims straight 

 forward, 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 

 Opalina. 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 chemio- 

 tactic 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 



