292 Darwin, and after Darwin. 



tinue 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 with- 

 drawal 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 fluc- 

 tuating about this average, unless for any reason the force of 

 heredity begins to fail in which case, of course, the average will 

 progressively fall in proportion to the progressive weakening 

 of this force. 



" Now, according to my views, the force of heredity under such 

 circumstances is always bound to fail, and this for two reasons. 

 In the first place, it must usually happen that when an organ 

 becomes useless, natural selection as regards that organ will not 

 only cease , but become reversed. For the organ is now absorbing 

 nutriment, causing weight, occupying space, and so on, uselessly. 

 Hence, even if it be not also a source of actual danger, ' economy 

 of growth ' will determine a reversal of selection against an organ 

 which is now not merely useless, but deleterious. And this de- 

 generating influence of the reversal of selection will throughout be 

 assisted by the cessation of selection, which will now be always 

 acting round a continuously sinking average. Nevertheless, 

 a point of balance will eventually be reached in this case, just as 

 it was in the previous case where the cessation of selection was 

 supposed to be working alone. For, where the reversal of selec- 

 tion has reduced the diminishing organ to so minute a size that 

 its presence is no longer a source of detriment to the organism, 

 the cessation of selection will carry the reduction a small degree 



