38 The Selection Theory 



they will then continue unobstructed, but with exceeding slowness, 

 along the downward path, until the organ becomes vestigial, and 

 finally disappears altogether. 



The fluctuations of the determinants hither and thither may thus 

 be transformed into a lasting ascending or descending movement ; 

 and this is the crucial point of these germinal processes. 



This is not a fantastic assumption ; we can read it in the fact 

 of the degeneration of disused parts. Useless organs are the only 

 ones ivhich are not helped to ascend again hy personal selection, and 

 therefore in their case alone can ive form any idea of how the 

 primary constituents behave, when they are subject solely to intra- 

 germinal forces. 



The whole determinant system of an id, as I conceive it, is in 

 a state of continual fluctuation upwards and downwards. In most 

 cases the fluctuations will counteract one another, because the passive 

 streams of nutriment soon change, but in many cases the limit from 

 which a return is possible will be passed, and then the determinants 

 concerned will continue to vary in the same direction, till they attain 

 positive or negative selection-value. At this stage personal selection 

 intervenes and sets aside the variation if it is disadvantageous, or 

 favours — that is to say, preserves — it if it is advantageous. Only 

 the determinant of a useless organ is uninfluenced by j^f^rsonal 

 selection, and, as experience shows, it sinks downwards ; that is, the 

 organ that corresponds to it degenerates very slowly but uninter- 

 ruptedly till, after what must obviously be an immense stretch of 

 time, it disappears from the germ-plasm altogether. 



Thus we find in tlie fact of the degeneration of disused parts the 

 proof that not all the fluctuations of a determinant return to equili- 

 brium again, but that, when the movement has attained to a certain 

 strength, it contijmes in the same direction. We have entire certainty 

 in regard to this as far as the downward progress is concerned, and 

 we must assume it also in regard to ascending variations, as the 

 phenomena of artificial selection certainly justify us in doing. If the 

 Japanese breeders were able to lengthen the tail-feathers of the cock 

 to six feet, it can only have been because the determinants of the 

 tail-feathers in the germ-plasm had already struck out a path of 

 ascending variation, and this movement was taken advantage of by 

 the breeder, who continually selected for reproduction the individuals 

 in which the ascending variation was most marked. For all breeding 

 depends upon the unconscious selection of germinal variations. 



Of course these germinal processes camiot be proved mathemati- 

 cally, since we cannot actually see the play of forces of the passive 

 fluctuations and their causes. We cannot say how great these fluctua- 

 tions are, and how quickly or slowly, how regularly or irregularly they 



