ENLARGEMENT OF PARTS FROM USE. 33 
by various causes in the water. Wyman has attempted 
to show that the semicircular canals are unusually large 
in the blind fish Amzblyopsis. 
The study of the examples of enlarged parts arising 
from increased use, additional blood supply, and irritation, 
teaches clearly enough that the same laws which regulate 
these processes under normal conditions are equally 
active under abnormal conditions, and indicates that the 
thick fur of mammals living in cold climate, or the local 
growth of hair on the skin of man when stimulated by 
irritants or unusual states of the nerves, are responses to 
such stimuli as call up the growth of hair in the stomach 
of the darter or crayfish ; the scales of serpents, feathers 
of birds, quills of porcupines, and bristles of hogs, are like 
hair, epidermis and horn, modifications of the surface 
epithelium, probably induced by variations in the nature 
of the stimuli, or irritants, and in differences of surround- 
ing conditions, such modifications being transmitted to 
the offspring. 
The inheritance of the effects of increased use of 
parts not only manifests itself in enlarged muscles, thick 
bones, and stout ligaments, but explains the large 
udders and bountiful supply of milk we obtain from 
domesticated cows, and, as Wallace rightly remarks, 
“almost perpetual egg-laying in poultry.” 
In a similar manner increased use of the special senses 
with the transmission of the extra acuteness gradually 
acquired by individuals, explains the wonderful power of 
scent in dogs, of sight in hawks, and of cunning in 
foxes. 
In a similar manner, also, the use of the fingers in 
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