118 THE PHILOSOPHY OF BIOLOGY 



like so to term them. Physics considers only the 

 statistical mean velocities and free paths. The irre- 

 versibility of physical phenomena, the fact that energy 

 tends to dissipate itself, the second law of thermo- 

 dynamics, depend on the assumption that Maxwell's 

 demons exist onl}'^ in imagination. We must appeal 

 to experience now. There is no a priori reason why 

 the phenomena of physics should be directed one way 

 and not the other, for it is possible to conceive a con- 

 dition of our Universe in which, for instance, solid 

 iron would fuse when exposed to the atmosphere. In 

 such conditions organisms would grow backwards from 

 old age to birth, with conscious knowledge of the future 

 but no recollections of the past. Experience shov>rs, 

 however, that phenomena do tend in one way — hut this 

 experience is that of experimental physics, so that for 

 the latter science Maxwell's demons do not exist. Now 

 physiology has borrowed from physics, not only the 

 experimental methods, but also the fundamental con- 

 cepts of thermodynamics. The organism, therefore (so 

 physiology must conclude) , cannot control the motions 

 of individual molecules, and so vital processes are 

 irreversible. But we have seen that the processes 

 of terrestrial life as a whole are reversible, or tend to 

 reversibility. We must therefore seek for evidence 

 that the organism can control the, otherwise, un-co- 

 ordinated motions of the individual molecules. 



The Brownian movement of very small particles 

 of matter is so familiar to the biologist that we need 

 not describe it. It is doubtless due to the impact of 

 the molecules of the liquid in which the particles are 

 suspended. Groups of molecules travelling at velocities 

 above the mean hit the particle now on one side, and 

 again on the other, and so produce the peculiar 

 trembling which Brown thought was life. Now the 



