On The Origin of Species by Charles Darwin Chapter 1 Page 15

(Lachnanthes), which coloured 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 have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.

Hence if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation.