Organic Evolution. 193 



Weismann likely to fall back upon any innate tendency to 

 degeneration. Unless, therefore, some cause be shown 

 why the negative variations should be prepotent over the 

 positive variations, we must, I think, allow that unaided 

 panmixia cannot affect any great amount of reduction. 



In this connection we may notice Professor Weismann's 

 newer view of the introduction of bodily mortality. He 

 says, " The problem is very easily solved if we seek 

 assistance from the principle of panmixia. As soon as 

 natural selection ceases to operate upon any character, 

 structural or functional, it begins to disappear. As soon, 

 therefore, as the immortality of somatic [body-] cells became 

 useless, they would begin to lose this attribute." * Even 

 granting that panmixia could continuously reduce the size 

 of ducks' wings, it is not easy to see how it could get rid of 

 immortality. The essence of the idea of panmixia is that, 

 when the natural selection which has raised an organ to 

 a high functional level, and sustains it there, ceases or is 

 suspended, the organ drops back from its high level. But 

 on Professor Weismann's hypothesis, immortality has 

 neither been produced nor is it sustained by natural selec- 

 tion. How, therefore, the cessation of selection can cause 

 the disappearance of immortality a character with which 

 natural selection has had nothing whatever to do Pro- 

 fessor Weismann does not explain. He seems to be using 

 "panmixia" in the same vague way that, in his previous 

 explanation, he used "natural selection." 



If panmixia alone cannot, to any very large extent, 

 reduce an organ no longer sustained by natural selection, 

 to what efficient cause are we to look ? Mr. Romanes has 

 drawn attention to the reversal of selection as distinguished 

 from its mere cessation. When an organ is being improved 

 or sustained by selection, elimination weeds out all those 

 which have the organ in an ill-developed form. Under a 

 reversal of selection, elimination will weed out all those 

 which possess the organ well developed. In burrowing 

 animals, the eyes may have been reduced in size, or even 



* Weismann, "Essay on Heredity," p. 140. 



O 



