220 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS; CORRELATED 

 VARIATION; INHERITANCE 



Changed habits produce an inherited effect, as in the period of the 

 flowering of plants when transported from one climate to another. 

 With animals the increased use or disuse of parts has had a more 

 marked influence; thus I find in the domestic 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 inherited 

 development of the udders in cows and goats in countries where 

 they are habitually milked, in comparison with these organs in other 

 countries, is probably another instance of the effects of use. Not 

 one of our domestic animals can be named which has not in some 

 country drooping ears; and the view, which has been suggested that 

 the drooping is due to disuse of the muscles of the ear, from the 

 animals being seldom much alarmed, seems probable. 



Many laws regulate variation, some few of which can be dimly 

 seen, and will hereafter be briefly discussed. I will here only allude 

 to what may be called correlated variation. Important changes in the 

 embryo or larva will probably entail changes in the mature animal. 

 In monstrosities, the correlations between quite distinct parts are 

 very curious; and many instances are given in Isidore Geoffrey St. 

 Hilaire's great work on this subject. Breeders believe that long limbs 

 are almost always accompanied by an elongated head. Some instances 

 of correlation are quite whimsical: thus cats which are entirely white 

 and have blue eyes are generally deaf; but it has been lately stated by 

 Mr. Tait that this is confined to the males. Color and constitutional 

 peculiarities go together, of which many remarkable cases could be 

 given amongst animals and plants. From facts collected by Heu- 

 singer, it appears that white sheep and pigs are injured by certain 

 plants, whilst dark-colored individuals escape: Professor Wyman has 

 recently communicated to me a good illustration of this fact; on ask- 

 ing some farmers in Virginia how it was that all their pigs were black, 

 they informed him that the pigs ate the paint-root (Lachnanthes), 

 which colored their bones pink, and which caused the hoofs of all but 

 the black varieties to drop off; and one of the "crackers "(i.e., Virginia 

 squatters) added, "we select the black members of a litter for raising, 

 as they alone have a good chance of living." Hairless dogs have 

 imperfect teeth; long-haired and coarse-haired animals are apt to 



