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 duck 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 
