250 THE EFFECT OF TEMPERATURE 



well-known instance of the direct action of light is 

 found in the bronzing of the human skin following on 

 undue exposure to the sun; but to what extent are we 

 entitled to refer the black skin of inhabitants of the 

 tropics to a similar, but more pronounced, action? 

 Eimer * is of the opinion that the effect is the direct 

 result of the more intense light and heat. Thus he 

 found that in passing down the Nile valley from the 

 Delta to the Soudan, the natives gradually became more 

 and more dark-skinned, the further south they lived. 

 The increased light and warmth, according to Eimer, 

 lead to a greater flow of blood to the skin, and the con- 

 sequent deposition of pigment. This effect is inher- 

 ited, and has become a constant character. There is, 

 of course, no warrant for laying down the law with such 

 assurance as this, for one can easily imagine several 

 other equally possible and plausible explanations to ac- 

 count for the facts. For instance, pigmentation may be 

 correlated with a greater resistance to the climate of 

 hot countries, or with greater physical strength, and 

 may have been increased by sexual selection. Still, 

 Eimer's explanation may contain a distinct modicum of 

 truth, and I hope to prove in a subsequent chapter 

 that the heritableness of acquired characters such 

 as increased or decreased pigmentation may be deduced 

 without assuming anything further than the present 

 state of knowledge legitimately warrants us in 

 doing. 



The diminution or disappearance of pigmentation fol- 

 lowing upon withdrawal of light is best illustrated by 

 reference to the well-known cave animals. Of these, 



* " Organic Evolution," p. 87. 



