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 Amblyopsis. 



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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