192 THE PHILOSOPHY OF BIOLOGY 



new, only as augmentations or diminutions of some- 

 thing previously existing ; and he regarded fluctuations 

 as due to the action of the environment, following in 

 their distribution the laws of chance. 1 Mutations, on 

 the other hand, were something quite new. Now 

 future analysis of variability will not, we think, bear 

 out the validity of this distinction. It is far more 

 likely that a fluctuation is a variation which is the 

 result of some causes the action of which is variable. 

 (We are regarding variability now as subject to " causa- 

 tion " in the physical sense, 

 for only by so regarding it 

 can we attempt its analysis) . 

 As a rule this process results 

 in a fluctuation, but if its 

 extent, or degree of opera- 

 tion, exceeds a certain " cri- 

 tical value " a mutation is 

 produced. We may, follow- 

 ing the example of the phys- 

 icists, illustrate this by a 

 " model." 



This model is a modifica- 

 tion of Galton's illustration of the degrees of stability 

 of a species. It is a disc of wood rolling on its 

 periphery. We divide it into sectors, and the arcs 

 ab, cd, ef, and gh have all the same radius, 10, 20, 

 30, and 40. Then we flatten the sectors be, de, fg, 

 and ha, so that their radii are greater than are those 

 of the other arcs. Now let us cause the disc to roll 

 about the point 8 as a centre. It will oscillate back- 

 wards and forwards about a mean position 8. Let us 

 think of these oscillations as fluctuations. 



Suppose, however, that we cause the disc to roll 



1 See Appendix, p. 351. 



