Chap. XIII. AND ABORTED ORGANS. 409 



have ultimately lost tlie power of flying. Again, an organ 

 useful under certain conditions, might become injurious under 

 others, as with tlic wings of beetles living on small and ex- 

 posed islands; and in tliis case natural selection would con- 

 tinue slowly to reduce the organ, until it was rendered harm- 

 less and ruihmentary. 



Any change in structure, and function, which can be ef- 

 fected by insensibly small steps, is within the power of natural 

 selection ; so that an organ rendered, through changed haliits 

 of life, useless or injurious for one purpose, might he modified 

 and used for another purpose. An organ might, also, be 

 retained for one alone of its f(jrmer functions. An organ, ori- 

 ginally formed by the aid of natural selection, when rendered 

 useless, may well be vai-iable, for its variations can no longer be 

 checked by natural selection. At whatever period of life either 

 disuse or selection reduces an organ, and this will generally 1)C 

 when the being has come to maturity and has to exert its full 

 powers of action, the principle of inheritance at corresponding 

 ages will reproduce the organ in its reduced state at the same 

 mature age, but will seldom affect it in the cmbrj-o. Thus we 

 can understand the greater size of rudimentary organs in the 

 embryo relatively to its other parts, and their lesser relative 

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

 wore to be inlierited, not at a corresponding age, but at a Aery 

 early perioil of life, the rudimentary part would tend to be 

 whoUv lost, and we should have a case of complete abortion. 

 Tlie principle, also, of the economy of organization, explained 

 in a former chapter, by which the materials forming any part, 

 if not useful to tlie possessor, will be saved as for as is pos- 

 sible, may often have come into play, and aided in the entire 

 obliteration of a rudimentary organ. 



As the presence of rudimentary organs is thus due to the 

 tendency in every part of the organization, which has long 

 existed, to be inherited — we can understand, on the genealo- 

 gical view of classification, how it is that systematists have 

 found rudimentary parts as useful as, or even sometimes more 

 useful than, parts of high ])hysiological importance. Kudimen- 

 tary organs may be comjiared with the letters in a word, still 

 retained in the spelling, but become useless in the pronuncia- 

 tion, but which serve as a clew for its derivation. On the view 

 of descent with modilication, we may conclude that the exist- 

 I'uce of organs in a rudimentary, imperfect, and useless condi- 

 tion, or quite al)orled, far from presenting a strange diilicultv, 

 18 



