1 84 SCIENCE PROGRESS 



uniformity of the water, so that, starting from one tube and 

 moving to the other, one passes through gradually diminishing 

 acidity, through neutrality, to a region of gradually increasing 

 alkalinity, which reaches a maximum at the orifice of the other 

 tube. The medium between the tubes, therefore, is accurately 

 graded in composition. 



Now let us see the effect of a trace of acid and alkali upon 

 these Vibrios. It is not always the same ; it depends upon the 

 particular forms we are examining. I choose the case where 

 acid slows the movements, alkali increases them. Each indi- 

 vidual Vibrio, as we watch it, is seen to move in an erratic and 

 irregular orbit, so erratic that we can consider it as completely 

 irregular. 



The problem now becomes a simple mathematical one. 

 Given a number of particles, each moving in an irregular orbit, 

 and uniformly distributed throughout a homogeneous medium. 

 The medium ceases to be homogeneous and is changed so that 

 in one region the mean velocity of each of the particles is 

 augmented, in another it is diminished. What will be the 

 effect upon the distribution of the particles ? The answer is, 

 that they will collect where the motion is slowest. 



We now try the experiment, and we find that the Vibrios 

 do collect where their motion is most slowed — namely, in the 

 region of maximal acidity. And they do not swim directly there ; 

 they, as it were, settle out in that region, as the hypothesis 

 demands. 



The influence of a chemically heterogeneous medium upon 

 the free cells living in it is called " chemiotaxis." I have 

 analysed a simple case, but it would take a session's lectures 

 to follow out the application of the principle to biological 

 problems. It explains great regions of disease, it has even 

 been applied to the workings of the nervous system. At one 

 sweep it embraces the directive effects of the surrounding 

 medium upon the movements of free cells, in the waters of the 

 earth and in the bodies of animals and of plants. 



The choice of food particles, the discrimination manifested 

 by Amoeba, is the chemiotactic response of its irregularly flowing 

 protoplasm to the chemical atmosphere, if I may so put it, of 

 the food particle. At the Mint a chance collection of sovereigns 

 are presented to a certain machine, and it sorts them into those 

 of full weight and those of short weight. A chance collection 



