APPENDIX I. 



ON PANMIXIA. 



THERE are several points of considerable theoretical im- 

 portance connected with Panmixia, which were omitted 

 from the text, in order to avoid distracting attention from 

 the main issue which is there under consideration. These 

 side issues may now be appropriately presented in the form 

 in which they were published in Nature, March 13, 1890*. 

 After stating, in almost the same words, what has already 

 been said in Chapter X, this paper proceeds, with the excep- 

 tion of a few verbal alterations, as follows. 



"There is, however, one respect in which Professor Weismann's 

 statement of the principle of panmixia differs from that which was 

 considered by Mr. Darwin ; and it is this difference of statement 

 which amounts to an important difference of theory that I 

 now wish to discuss. 



" The difference in question is, that while Professor Weismann 

 believes the cessation of selection to be capable of inducing de- 

 generation down to the almost complete disappearance of a rudi- 

 mentary organ, I have argued that, unless assisted by some other 

 principle, it can at most only reduce the degenerating organ to 

 considerably above one-half its original size or probably not 

 through so much as one-quarter. The ground of this argument 

 (which is given in detail in the Nature articles of 1873-1874) is, 

 that panmixia depends for its action upon fortuitous variations 

 round an ever-diminishing average the average thus diminish- 

 ing because it is no longer sustained by natural selection. But 

 although no longer sustained by natural selection, it does con- 

 1 Vol. xli. p. 438. 

 U 2 



