40 GENETICS 



non-heritable fluctuating modifications. They all hold 

 predictable relations to the average position shown 

 when the pendulum comes to rest, because whenever 

 the pendulum is put in motion the various positions 

 all recur as before. A mutation, on the contrary, is 

 represented by a change in the point of attachment at 

 the upper end of the pendulum. It occurs only when 

 the entire pendulum is unhooked and hung up in a 

 different place. This new point of attachment must be 

 chosen arbitrarily and has no such definite relation to 

 the original attachment as characterizes the variation 

 in position of the swinging end of the pendulum. 



When the attempt is made to arrange a series of suc- 

 cessive mutations in a curve they do not show a graded 

 relationship to each other as fluctuations do. The 

 latter mass around the average standard according to 

 the laws of chance in much the same way that a hundred 

 shots by a good marksman may center around a bull's- 

 eye. Mutations never group in this way. They find no 

 correspondence even with wild shots at the bull's-eye. 

 They are shots directed at a different target altogether. 

 To use the musician's phraseology, a variation elabo- 

 rated upon an old theme would correspond to a modi- 

 fication but a mutation would be an entirely new 

 theme. 



Darwin was fully aware of the existence of mutations 

 or "sports" as he called them, and incidentally gave 

 time to their consideration, but the great task which 

 he set out to accomplish in such a masterly manner 

 was to overthrow the widespread and deep-seated be- 

 lief of his day in a sudden special creation of distinct 



