328 THE POPULAR SCIENCE MONTHLY 



to show that the birds when adults exhibit any variation as a result of 

 all these manipulations. 



It is noticeable that the range of temperature (within which any 

 growth is possible) is more restricted for birds than it is for the lower 

 animals. 



The comparative difficulty of producing a modification in birds is 

 exemplified, in another interesting way, in the elasticity which the hen 

 possesses of producing a shell when carbonate of lime is absent. This 

 was referred to earlier in this article when commenting on the modifi- 

 cation produced in crab-shells. 



Eegenerative powers, as is well known, are very slight in birds. In 

 two forms at least, the stork and the fighting cock, the beak will re- 

 generate. This fact 35 has been discussed by Weismann and others in 

 connection with the theory of " regeneration and liability to injury," 

 but it does not appear to have been noted that the beak is an integu- 

 mentary structure and that of all tissues the epidermis is one of the 

 easiest to modify. Neither the wings nor feet of birds will regenerate. 



Mammals 



In mammals, as in birds, the chief modifications are concerned 

 with the skin and its appendages. 



There is good evidence that changes in climatic surroundings directly 

 affect the color of the hair of some of the mammalia, though at the 

 same time it is evident that others remain unchanged. To a certain 

 extent the white winter coat of the Hudson Bay lemming, and changes in 

 the coloration of hares and rabbits, must be due to direct influence of 

 temperature. 36 Many arctic animals, however, do not change their 

 coat color with the season. Changes in the amount and quality of the 

 hair of various quadrupeds on transportation from one part of the 

 world to another are abundantly recorded. 37 



Other well-known modifications associated with the integument 

 are thickening of the human epidermis by pressure and friction, and 

 darkening of the skin by the action of the sun's rays. The effect of 

 sunlight on the higher animals appears, however, in regard to the vital 

 functions, to be merely superficial. We have, for example, many in- 

 stances where prisoners have spent long lives in darkness or have per- 

 haps been freed after years of confinement and have then resumed 

 their normal activities. Working mules have been kept in mines 

 for long periods of time, as much as twenty years, " and beyond tem- 

 porary sensitiveness of the eyes no effect was perceptible." 3 



35 Morgan, " Regeneration," pp. 95, 97, 106. 



39 Vernon, pp. 243, 330, 331. Morgan, "Exp. Zool.," p. 13. 



37 De Varigny, pp. 88-91. 



3S E. Davenport, " Principles of Breeding," p. 244. 



'38 



