34 DA R W1NISM AND RA CE PRO CRESS. 



operation of selection in such instances, nor have 

 we any reason for saying that part of the effect 

 must be due to some other cause. In the case of 

 organs which become useless and finally disappear in 

 the course of generations, a selective agency will 

 sufficiently account for this disappearance. As 

 Darwin himself pointed out, a useless organ is an 

 expense and a drain upon an animal's capital; it 

 requires blood, and its exercise uses up some of the 

 sum total of energy the animal possesses. The truth 

 of this can be shown experimentally, as when com- 

 pensatory growth occurs in the rest of the body after 

 amputation of a limb, or when one lung or kidney 

 enlarges subsequently to the disease or removal of the 

 other. In cases where an organ is useless, those who 

 have it badly developed, and in consequence have 

 other and useful parts more fully formed, will have 

 a distinct advantage over those born with a well- 

 formed but useless organ. We may thus explain the 

 small size of the wings of the tame as compared with 

 the wild duck, an instance in which Darwin saw 

 difficulty in excluding inherited disuse. In this way 

 we may explain the occurrence of the still smaller 

 wings of the running ostrich land apteryx ; also the 

 blind fish of the Kentucky Cave, and the visionless 

 eyes of the burrowing mole. As an illustration, an 

 animal or man may, in this respect, be compared to 

 an individual with a given amount of capital, who, 



