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; péebaldness, 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; polydac- 
iylism, 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; Frzedreich’s disease, degeneration of the upper part of the spinal 
cord; Ménzére’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; hemophilia, ex- 
cessive bleeding from wounds; myopza, near-sightedness; multiple sclerosis; 
neuritis optica, progressive atrophy of the optic nerve; Gower’s muscular 
atrophy; night-blindness, some forms; ichthyosis, a peculiar scaly earings 
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 
Digitized by Microsoft® 
