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 Geoffroy 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 
