142 THE ACTION OF THE LIVING CELL 



degree of protection from the elements. Although this 

 is by no means a perfect protection, nature has en- 

 dowed the dermal cells with the ability to fortify 

 themselves, and consequently the more delicate under- 

 lying tissues, when properly stimulated. To this end 

 continual exposure of the skin to mechanical friction 

 results in the formation of callus. The reader will 

 readily recall other obvious examples of such de- 

 vices, by which an animal or plant is enabled to cope 

 with limited variations in an environment to which 

 it is not perfectly adapted. 



It will be noted that whereas the mechanism of the 

 increased development of resistance to sunburn is 

 readily understood on the basis of the increased for- 

 mation of pigment, no such simple explanation is 

 forthcoming to account for the formation of callus. 

 In the majority of cases, the mechanism underlying an 

 observed increase in natural resistance is not in the 

 least obvious. Similarly, while individual variations 

 in natural resistance are apparent at all times, we have 

 little exact knowledge concerning the underlying 

 causes of such variation. To illustrate : When a series 

 of animals receive identical wounds inflicted by the 

 same means and in the same manner, the various in- 

 dividuals respond dififerently. In some, the wounds 

 heal uneventfully; in others, they may heal slowly; 

 and in some individuals shock and other untoward 

 disturbances are experienced. To what may such dif- 

 ferences in reaction be attributed? We may answer, 

 to individual differences in natural resistance. Quite 

 obviously, we cannot logically postulate a natural re- 

 sistance to the particular type of wound inflicted, or 

 the particular device employed to produce the injury. 



