TRANSFORMISM 249 



that it should be perpetuated, in the conditions of wild nature. 

 That is, a new category of organisms should have appeared and 

 some agency must have caused this new category to persist. 



85. ON THE ''CAUSES'' OF MUTATIONS 



Scientific method seems to oblige us to seek for the '' causes," 

 or antecedents, or conditions of appearance of mutations. We 

 may best approach this problem by first considering more in 

 detail what is meant by fluctuations and we are forced to investigate 

 the matter from the purely physical side. 



85a. The Multiple Values of a Characteristic. We take 

 first, the most convenient model of a physical system, a small 

 volume of gas at some constant volume, temperature and pressure. 

 The individuals of the system are molecules and a characteristic 

 of these individuals is their velocity. Each molecule moves with 

 a certain speed until it collides with another molecule or with 

 the walls of the vessel : then in general the speed changes. There 

 will be a mean molecular velocity and a certain fraction of all 

 the individuals will be moving with a speed that is a little less, 

 or a little greater than this. Call the mean velocity v then the 

 range v :^ e will be that at which this fraction of all the molecules 

 move. Above and below this range other molecules are moving, 

 that is, there are fluctuations of velocity. There will also be 

 fluctuations of pressure upon any small area of the wall of the 

 container. We assume that all the molecules (say of hydrogen, 

 H2) are similar to each other in respect of mass, but there is really 

 no reason for this assumption. All we know is the mean mass of 

 a great number of hydrogen molecules and it is probable (or it 

 is just as reasonable to assume) that the mass of the individual 

 molecules fluctuates about a certain mean value. 



What we have said applies also to any other measurable physical 

 characteristic as observed in inorganic individuals — that is 

 electrons, atoms, molecules, crystals, colloidal particles, etc. 

 The characteristic (mass, speed, form, etc.) has a mean value 

 and there are fluctuations in the individuals from this mean value. 

 We do not easily observe such fluctuations in the cases of atoms 

 and molecules since these are so small that what we always infer 

 is a statistical result — the mean of a great number of individual 

 effects. Still we can conclude that every physical result has 



