410 



TRANSMISSION 



limbs ; the wings have gone from the apteryx and appear to be 

 going from the ostrich ; eyes of cave insects and fishes are in 

 many cases imperfect or rudimentary ; horses, cattle, sheep, 

 and hogs have lost toes that belonged to their ancestors, and 

 parts generally which are functionless are evidently disappearing. 

 How, now, are parts lost, when once they have become useless ? 



Economy of Nature not the reason for loss of parts. It is 

 sometimes said that a part no longer used is removed by Nature 

 in the interest of economy. This is bad science. Nature is not 

 economical. She not only supports many expensive and useless 

 parts, such as tremendous horns and tusks, but she often pro- 

 duces necessary products in wanton profusion, such as pollen 

 and fat. Other and deeper causes are at work, causes more 

 rationally connected with the facts of life, than any such 

 anthropomorphic reason as economy. Whatever may be true 

 as to economy of growth, it is a fact, not a principle ; a result, 

 not a cause. 



How parts disappear. We are reasonably intelligent upon a 

 part of the process of degeneracy and disappearance, at least in 

 the more active portions of the body. As long as a part is in 

 use, its constant movements increase the flow of blood to that 

 part and it enjoys the extreme development that comes only with 

 maximum nourishment and uniform healthy exercise. When, 

 however, the part is no longer used, the flow of blood is lessened 

 and it suffers from lessened nourishment. In this way the first 

 steps of degeneration are easily accounted for. 



If the part has been useful heretofore, it has of course been 

 sustained by selection. Being no longer useful, this influence is 

 withdrawn, and breeding is henceforth totally without reference 

 to this particular part; that is to say, there is absolute "cessa- 

 tion of selection," or panmixia, 1 as to this part. Under this con- 

 dition the average parentage would be lower than heretofore, thus 

 accounting for a st\\\ farther step in the downward process. 



To all this is often added the adverse effect of selection, 

 when for any reason that influence is turned against the part. 

 This " reversal of selection " of course comes only when a part 

 once useful has become not merely useless but detrimental. In 



1 Romanes, Darwin and After Darwin, II, 97-100, 291-306. 



