188 THE BREEDING OF ANIMALS 



tions, called " hormones/' emanate from various glands 

 and perhaps from most of the internal organs. If the 

 thyroid and parathyroid bodies are removed from the 

 body, death follows. The destruction of the pituitary 

 glands in man causes the bones of the hands, feet and jaws 

 to enlarge (gigantism), causing death. 



It is probable that the milking function in the domes- 

 tic animals has some connection with the activities of a 

 specific " hormone " which is essential. 



177. Sex-linked characters. Certain characters are 

 so closely related to sex that their transmission is 

 influenced by such relation. These characters have 

 been called sex-linked or sex-limited characters. They 

 are to be distinguished from secondary sexual characters. 



178. Color-blindness. Men are much more fre- 

 quently color-blind than women. Color-blind men do 

 not transmit color-blindness directly to sons, but to grand- 

 sons through their daughters. The daughters of color- 

 blind men are not themselves color-blind, but tend to 

 transmit this deficiency to their sons. Color-blindness 

 in the daughter could be produced only when the father 

 was color-blind and the mother possessed the power to 

 transmit color-blindness. Color-blindness is the result of 

 some defect in the germ-cell. This factor which is asso- 

 ciated with an x chromosome appears twice in the ovum 

 and only once in the sperm. A similar condition is found 

 in the pomace fly 1 (Drosophila ampelophila). This form 

 has normally red eyes, but this apparently is a unit char- 

 acter, sex-linked in transmission. An interesting case 

 of sex-linked heredity is found in the Barred Plymouth 

 Rock fowl. 2 Pure barred fowls when mated produce 



1 Castle, "Heredity and Eugenics," p. 75. 



2 Castle, "Heredity," 1911, p. 170. 



