Chap XXIV. ARRESTED DEVELOPMENT AND RUDIMENTS. 309 



repeatedly under Nature is shown by the many gradations 

 which exist between organs in a perfect state and the merest 

 vestiges of them. Mr. Romanes 89 has, I think, thrown much 

 light on this difficult problem. His view, as far as it can be 

 given in a few words, is as follows : all parts are somewhat 

 variable and fluctuate in size round an average point. Now, 

 when a part has already begun from any cause to decrease, it 

 is very improbable that the variations should be as great in 

 the direction of increase as of diminution; for its previous 

 reduction shows that circumstances have not been favourable 

 for its development ; whilst there is nothing to check varia- 

 tions in the opposite direction. If this be so, the long con- 

 tinued crossing of many individuals furnished with an organ 

 which fluctuates in a greater degree towards decrease than 

 towards increase, will slowly but steadily lead to its diminu- 

 tion. With respect to the complete and absolute abortion of 

 a part, a distinct principle, which will be discussed in the 

 chapter on pangenesis, probably comes into action. 



With animals and plants reared by man there is no severe 

 or recurrent struggle for existence, and the principle of 

 economy will not come into action, so that the reduction of 

 an organ will not thus be aided, So far, indeed, is this from 

 being the case, that in some few instances organs, which are 

 naturally rudimentary in the parent- species, become partially 

 redeveloped in the domesticated descendants. Thus cows, 

 like most other ruminants, properly have four active and two 

 rudimentary mammae ; but in our domesticated animals, the 

 latter occasionally become considerably developed and yield 

 milk. The atrophied mammae, which, in male domesticated 

 animals, including man, have in some rare cases grown to full 

 size and secreted milk, perhaps offer an analogous case. The 

 hind feet of dogs naturally include rudiments of a fifth toe, 

 and in certain large breeds these toes, though still rudimentary, 



89 I suggested in 'Nature' (vol. viii. natural selection would, owing to 



pp. 432, 505) that with organisms intercrossing, slowly but steadily 



subjected to unfavourable conditions decrease. In three subsequent com- 



all the parts would tend towards munications to 'Nature' (March 12, 



reduction, and that under such cir- April 9, and July 2, 1874), Mr. 



cumstances any part which was not Romanes gives his improved view, 

 kept up to its standard size by 



