NATURAL RESISTANCE AND FATIGUE 141 



the individual has not been previously forced to con- 

 tend with. As a rule such defense mechanisms do not 

 appear instantly when needed, but are developed as 

 necessary from some latent source when the organ- 

 ism is appropriately stimulated by an environmental 

 factor. As a simple instance of this in man, let us 

 briefly consider the phenomena incident to sunburn. 

 We are all familiar with the fact that after a winter 

 indoors the exposure of our skin to the bright summer 

 sun for a sufficiently long period usually results in a 

 painful sunburn. If, however, we submit to a series 

 of such exposures, of lesser duration, the skin becomes 

 "tanned," after which prolonged exposure to the sun 

 does not result in a burn. This is due to the fact that 

 while our skin contains more or less pigment which 

 hinders the absorption of solar radiation, the concen- 

 tration of this pigment ordinarily is not sufficiently 

 great to prevent serious consequences from a single 

 prolonged exposure. On the other hand, the series of 

 short exposures necessary to elicit a good tan stimu- 

 lates the formation of an increased amount of the 

 protective pigment within the skin. In this way, then, 

 the human body can protect itself against highly in- 

 tense solar radiation. Due to the slight amount of pig- 

 ment normally present within the skin, all humans 

 possess to some degree a natural resistance to sunlight, 

 but, as the phenomena of tanning shows, this natural 

 resistance may be intensified by the suitable stimula- 

 tion of pigment formation — i.e., of a latent natural 

 resistance. 



The development of callus of the skin as a pro- 

 tection against a mechanical injury may be analyzed 

 in a similar fashion. Our skin affords us a certain 



