THE PHYSICAL BASIS OF LIFE 183 



each of which breeds true. They manifest that property of 

 living matter called heredity. 



Lastly, the individual Amoeba is in incessant movement, and 

 with each successive generation there is growth of individuals 

 and increase in the mass of living matter. Now, to these three 

 features, choice and purpose, groivtJi, and heredity, I propose to 

 confine myself, and I will consider them separately and in the 

 order named. 



The Faculty of Choice 



In one of Jules Verne's books, which at one time or another 

 held most of us in thrall, there is an account of a submarine 

 vessel which for a long time was conjectured to be some mighty 

 marine monster. Now, I want you to put yourself in a similar 

 position with respect to a model Whitehead torpedo, to con- 

 sider yourself as meeting one of these for the first time and 

 studying its movements under the impression that it is a 

 living being. The torpedo must be without its charge, other- 

 wise your experiments would come to an abrupt end ; for I 

 wish you to consider yourselves as inquiring why this curious 

 beast should always swim at the same depth. Push it down, 

 pull it to the surface, it would presently be swimming again as 

 many inches below the surface as before, and you would say to 

 yourselves, Why on earth does it choose to swim there ? 



Instead of a model torpedo, here in a drop of fluid are 

 countless thousands of the most minute forms of life, each 

 actively darting hither and thither, each so small that 5,000 

 would make only one large Amoeba. Into that drop you intro- 

 duce two fine capillaries, the one filled with very dilute acid, 

 the other with very dilute alkali, and in no very long time you 

 will find that the Vibrios have collected in a mass at one or 

 other of the tubes — probably the acid tube. If you followed 

 ordinary usage, you would express the result again in terms of 

 choice by saying that the animals are attracted to the acid and 

 are repelled from the alkali. 



Both cases illustrate the difficulty in freeing the imagination 

 from the tyranny of the counters it employs. The first case, 

 that of the 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 



