THE INDIVIDUAL AND THE SPECIES 193 



a little more violently, so that it oscillates until either 

 of the points 3 or 4 are perpendicularly beneath the 

 centre 0. In either of these positions the disc is in a 

 condition of ' unstable equilibrium," and an infini- 

 tesimal increase in the extent of an oscillation will 

 cause it to roll beyond the points 3 or 4. But if it 

 does pass either of these critical points it will begin 

 to oscillate about either of the new centres 5 or 7, thus 

 rolling on one of the arcs, ha or de. This assumption 

 of a new condition of stability we may compare with 

 the formation of a mutation. 



All this is merely a conceptual physical model of 

 a process about which we know nothing at all. It is 

 meant to illustrate the view that the organisation of 

 a plant or animal is not something absolutely fixed 

 and invariable. The organism in respect of each 

 recognisable and measurable character oscillates 

 about a point of stability, that is to say exhibits 

 fluctuating variations about the mean value of this 

 character. If the stability of the organisation is 

 upset, so that it oscillates, or fluctuates about a 

 new centre, that is, if the variations deviate in either 

 direction from a new " type ' or mean, a mutation 

 has been established. A mutation is not, therefore, 

 necessarily a large departure from ' normality." It 

 is not necessarily a " discontinuous variation," nor a 

 " sport " nor a " freak." It is essentially a shifting 

 of the mean position about which the variations 

 exhibited by the organism fluctuate. 



Such a mutation will, in general, involve the 

 creation of an " elementary species." We have con- 

 sidered only one character, say stature, in the above 

 discussion, but it generally happens that the assump- 

 tion of a new centre of stability involves all the char- 

 acters of the mutating organism. An elementary 



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