512 HUMAN BIOLOGY 



3. Individuals of the human species vary widely in 

 their susceptibiHty to this injury. These differences are 

 largely dependent directly on the surface pigmentation 

 although it is not unhkely that other quahties of the skin 

 may be of definite influence. 



4. All of the diff"erences in pigmentation which are 

 commonly recognized as characteristic of the various races 

 and intraracial types of the human species are of significance 

 for this disease. The most highly colored race (the negro) is 

 supposed to be absolutely, and doubtless is practically, 

 insusceptible to this injury or, in other words, possesses a 

 complete natural immunity. The most completely blond 

 types are most susceptible. Since there is entirely satisfying 

 evidence that the pigmentary variations are controlled 

 by the inheritance in accordance with Mendehan principles 

 it may with propriety be said that susceptibility to 

 sunburn (or per contra, natural immunity against it) is 

 inherited. 



5. Many of the less extreme blonds and all of the lighter 

 grades of the positively pigmented types develop increased 

 pigmentation (tan) under repeated exposure. The capacity 

 for tanning varies enormously and many rather complete 

 blonds seem to be entirely lacking in it. The tanning is in 

 effect an acquired immunity to a specific injury. And it is 

 proper to say that many individuals who under ordinary 

 conditions are very susceptible to sunburn may by treatment 

 be given a very perfect immunity against it, while others are 

 not only naturally very susceptible but lack certain capacities 

 and consequently cannot be rendered immune. From the 

 point of view of the inheritance we are here concerned with 

 the same mechanism that was considered in the preceding 

 paragraph, i.e., the inherited pigmentary control, and we may 

 accordingly consider that we have not only an inherited 

 natural immunity but a variable inherited capacity to 

 acquire an artificial immunity. 



6. The natural pigmentation is transmitted as a blending 

 type of inheritance, that is, it is controlled by multiple unit 

 characters in the Mendelian sense. The same may naturally 

 be said of the susceptibility to, and the capacity to acquire an 

 immunity against, sunburn. 



7. Finally, not to strain the illustration it may be pointed 



