Chap. XIII. RUDIMENTARY ORGANS. 455 



former functions. An organ, when rendered useless, 

 may well be variable, for its variations cannot be 

 checked by natural selection. At whatever period of 

 life disuse or selection reduces an organ, and this will 

 generally be when the being has come to maturity and 

 to its full powers of action, the principle of inheritance 

 at corresponding ages will reproduce the organ in its 

 reduced state at the same age, and consequently will 

 seldom affect or reduce it in the embryo. Thus we can 

 understand the greater relative size of rudimentary 

 organs in the embryo, and their lesser relative size in 

 the adult. But if each step of the process of reduction 

 were to be inherited, not at the corresponding age, but 

 at an extremely early period of life (as we have good 

 reason to believe to be possible) the rudimentary part 

 would tend to be wholly lost, and we should have a case 

 of complete abortion. The principle, also, of economy, 

 explained in a former chapter, by which the materials 

 forming any part or structure, if not useful to the pos- 

 sessor, will be saved as far as is possible, will probably 

 often come into play ; and this will tend to cause the 

 entire obliteration of a rudimentary organ. 



As the presence of rudimentary organs is thus 

 due to the tendency in every part of the organisation, 

 which has long existed, to be inherited — we can under- 

 stand, on the genealogical view of classification, how it is 

 that systematists have found rudimentary parts as useful 

 as, or even sometimes more useful than, parts of high 

 physiological importance. Eudimentary organs may be 

 compared with the letters in a word, still retained in 

 the spelling, but become useless in the pronunciation, 

 but which serve as a clue in seeking for its derivation. 

 On the view of descent with modification, we may con- 

 clude that the existence of organs in a rudimentary, 

 imperfect, and useless condition, or quite aborted, far 



