i^o2.] Ellioi. Iiilicrittuicc of Acquired C/niraciers. Ci'i 



Panmixia, but Prof. Romanes terms it Cessation of Selection. 

 The first-nameil author contends that this cessation of selection 

 is capable of inducing' degeneration down to the almost complete 

 disappearance of a rudimentary organ, but Prof Romanes argues 

 that unless assisted by some other principle it can at most only 

 reduce the degenerating organ to considerably above half or even 

 one quarter its original size ; because although no longer sus- 

 tained by natural selection, it is by heredity, and as long as this 

 force is unimpaired, the withdrawal of selection cannot reduce 

 the organ much below the efficient level above wdiich selection 

 maintained it. But he farther argues that the force of heredity 

 must fail, because a useless organ absorbs nutriment, occupies 

 space, etc., and then natural selection not only ceases, but be- 

 comes reversed, and hastens the degeneration of the organ until a 

 point of balance is reached, and the organ being no longer a 

 source of detriment remains as a rudiment, and so it would re- 

 main forever if heredity were everlasting, which is not reasonable 

 to suppose, and that the eventual disappearance of the organ 

 would be caused by this failure of heredity. Prof. Weismann at 

 first argued that the degeneration of an organ from disuse could 

 be etiected by panmixia alone, until it was reduced to five per 

 cent of its original size, or in his words* "the complete disap- 

 pearance of a rudimentary organ can only take place by the 

 operation of natural selection ; this principle will lead to its 

 elimination, inasmuch as the disappearing structure takes the 

 place and the nutriment of other useful and important organs ;" 

 not that the organ is transmitted from parent to ofispring in a 

 continued diminished state until it disappears, not from a failure 

 of heredity itself, but by the influence of selection exerting a dete- 

 riorating etiect. This he modifies, however, in his reply to 

 Prof. Vine's criticismf when he says, "organs in disuse become 

 rudimentary, not through the direct action of disuse, but because 

 natural selection no longer sustains them." Therefore these dis- 

 used organs which have become 'acquired characters,' trans- 

 mitted from parent to ofispring through undiminished conditions, 

 disappear not through the failure of heredity but by selection 

 withholding its sustaining power, or as Prof. Romanes has ar- 

 gued, acting in an opposite degree. 



*Essays, 2d ed. p. 89. 

 tNature, Oct. 24, 1889. 



