398 Mr. William Bate Hardy [April 6, 



torpedo, has served its purpose as an illustration, and it interests us no 

 longer. Let us see whether a purely mechanical conception will explain 

 the second. 



The acid and alkali diffusing out of the tubes destroy the 

 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 composi- 

 tion. 



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 individual 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 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 dimin- 

 ished. What will be the effect upon the distribution of the particles ? 

 The answer is that tliey 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 ex])lains 

 great regions of disease, it has even been applied to the workings of 

 the nervous system. At one sw^ep it embraces the directive effects 

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

 waters of tlie 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 of particles 

 are presented to amcelia, and it sorts tliem very im})erfectly into 

 tliose which modify the incessant streaming of its protoplasm so that 

 they become engulphed, and tliose 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 nmch less perfect here than 

 in tlie case of the vibrios, Ijccause the oscillations — t)ie movements of 



