There are several points of considerable theoretical importance 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[140]. After stating, in almost the same words, what has already been said in Chapter X, this paper proceeds, with the exception of a few verbal alterations, as follows. [140] Vol. xli. p. 438. "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 degeneration down to the almost complete disappearance of a rudimentary 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 diminishing because it is no longer _sustained_ by natural selection. But although no longer sustained by _natural selection_, it does continue to be sustained by _heredity_; and therefore, as long as the force of heredity persists unimpaired, fortuitous variations alone--or variation which is no longer controlled by natural selection--cannot reduce the dwindling organ to so much as one-half of its original size; indeed, as above foreshadowed, the balance between the positive force of heredity and the negative effects of promiscuous variability will most likely be arrived at above the middle line thus indicated. Only if for any reason the force of heredity begins to fail can the average round which the cessation of selection works become a progressively diminishing average. In other words, so long as the original force of heredity as regards the useless organ remains unimpaired, the mere withdrawal of selection cannot reduce the organ much below the level of efficiency above which it was previously _maintained_ by the _presence_ of selection. If we take this level to be 80 or 90 per cent. of the original size, cessation of selection will reduce the organ through the 10 or 20 per cent., and there leave it fluctuating about this average, unless for any reason the force of heredity begins to fail--in which case, of course, the average will