94 Evolution and Adaptation 



herited effect of " habit and of the use and disuse of parts," 

 or what is generally known as the Lamarckian factor of 

 heredity. Darwin believes that changes in the body of the 

 parent, that are the result of the use or of the disuse of a part, 

 may be transmitted to the descendants, and cites a number 

 of cases which he credits to this process. As we shall deal 

 more fully with this topic in another chapter, we may treat it 

 here quite briefly. As an example of the inheritance of dis- 

 use, Darwin gives the following case : " I find in the domes- 

 tic duck that the bones of the wing weigh less and the bones 

 of the leg more in proportion to the whole skeleton than do 

 the same bones in the wild duck, and this change may be 

 safely attributed to the domestic cluck flying much less and 

 walking more than its wild parents." The great and in- 

 herited development of the udders of cows and of goats in 

 countries where they are habitually milked, in comparison 

 with these organs in other countries, is given as another 

 instance of the effect of use. " Not one of our domestic 

 animals can be named that in some country has not drooping 

 ears, and the view has been suggested that the drooping is 

 due to the disuse of the muscles of the ears from the animals 

 being seldom much alarmed." 



It need scarcely be pointed out here, that, in the first case 

 given, those ducks would have been most likely to remain in 

 confinement that had less well-developed wings, and hence 

 at the start artificial selection may have served to bring 

 about the result. The great development of the udders of 

 cows and of goats is obviously connected with the greater 

 milk-giving qualities of these animals, which may have been 

 selected for this purpose. 



Another " law " of variation recognized by Darwin is what 

 is called correlated variation. For example, it has been 

 found that cats which are entirely white and have blue eyes 

 are generally deaf, and this is stated to be confined to the 

 males. The teeth of hairless dogs are imperfect; pigeons 



