DISEASE AND RELATED PHENOMENA IN ANIMAL BREEDING 525 



insipidus; diabetes mellitus; muscular atrophy of the ordinary type; 

 glaucoma, internal swelling and pressure on the eyeball; displaced 

 lens; colomba, an open suture in the iris; piebaldness, spotting of the dermal 

 coat; corneal opacity; Huntington's chorea, a disease similar to St. Vitus' 

 dance, a dangerous malady which first exhibits itself in middle life; 

 retinosa pigmentosa, pigmentary degeneration of the retina; polydao 

 tylism, extra fingers and toes; syndactylism, fusion of digits; congenital 

 cataract; hemeralopia, hereditary night-blindness; and brachydactylism, 

 shortening of the digits. A corresponding list of recessive defects includes 

 susceptibility to cancer; chorea, St. Vitus' dance; true dwarfism in which 

 all parts are proportionally reduced; alkaptonuria, urine darkens after 

 passage; alcoholism and criminality when based on mental deficiency, 

 hereditary hysteria; multiple sclerosis, diffuse degeneration of nervous 

 tissue; Friedreich's disease, degeneration of the upper part of the spinal 

 cord; Meniere's disease, dizziness and roaring in the ears; Thomson's 

 disease, lack of muscular tone ; hereditary ataxia; possibly the tendency 

 to become hard of hearing in advanced age; possibly non-resistance to 

 tuberculosis; feeble-mindedness of various types. In man the sex- 

 linked defects include Daltonism, color-blindness characterized by 

 inability to distinguish between reds and greens; haemophilia, ex- 

 cessive bleeding from wounds; myopia, near-sightedness ; multiple sclerosis ; 

 neuritis optica, progressive atrophy of the optic nerve; Gower's muscular 

 atrophy; night-blindness, some forms; ichthyosis, a peculiar scaly condition 

 of the skin; syndactyly, some forms. 



When the short time in which true systematic attention has been 

 given to the inheritance of human defects is recalled, it can be seen 

 that this even now incomplete list is truly a formidable array of infirmi- 

 ties. Commonly of course prejudice against those individuals which 

 display noticeable defects is so great that they have less opportunity 

 for reproduction than the more fit members of society. This is particu- 

 larly true of the more serious, dominant defects, and there is a consequent 

 tendency for these to run out. Some very serious dominant defects, how- 

 ever, such as Huntington's chorea are commonly not exhibited until 

 late in life after the common reproductive period is passed. It is 

 difficult, therefore, to state in such cases which individuals should and 

 which should not be permitted to bring forth offspring. In the case of 

 recessive defects, the factor may be carried by normal individuals, and 

 may therefore continually crop out among the progeny of normal parents. 

 To breed out defects in general, therefore, it is necessary to reject all 

 defectives for breeding purposes, and to mate all individuals from de- 

 fective stock, if at all, to stocks known not to be defective with respect 

 to the infirmity in question. A word of warning is necessary here on 

 account of the nature of our knowledge of human heredity. The 



