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 modification produced in crab-shells.
Regenerative powers, as is well known, are very slight in birds. In two forms at least, the stork and the fighting cock, the beak will regenerate. This fact[1] 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 integumentary 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.[2] 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.[3]
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 instances where prisoners have spent long lives in darkness or have perhaps 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 temporary sensitiveness of the eyes no effect was perceptible."[4]
