130 YORKSHIRE NATURALISTS UNION. 



diffused light is of a particular but unascertained actinic quality, 

 that the skin of a European is sunburnt.' Mr. Winwood 

 Reade * remarks that the negroes are blackest where the 

 climate is dampest, those away from the swamps inclining more 

 to brown. Mr. Edward D. Blyden, Professor in Liberia 

 College, Monrovia, t states that mulattoes in that country are 

 very frail and incapable of continued exposure or exertion. If, 

 as Tyndall points out, the ultra-violet rays have the strongest 

 chemical effect upon living organisms, these experiences would 

 tend to prove that what is known as sunburning is owing to 

 chemical action ; for the ultra-violet rays would be less inter- 

 cepted by rain-clouds or mist than would the light-giving rays 

 of the solar spectrum. In any case it would at first sight appear 

 that the dark-skinned man is less adapted for the conditions of 

 existence in a tropical climate than is the white-skinned man. 

 But this is opposed to all experience. It is well known that a 

 native can go on working in tropical sunshine where a white 

 man would be by heat quite incapacitated for exertion. The 

 simple explanation which suggests itself is : that since the rate 

 at which bodies radiate or part with heat is equivalent to that 

 at which they absorb it, the capability for rapid relief is pro- 

 portionate to the increased susceptibility ; and if the advantages 

 gained by the power of rapid radiation outweigh the disadvan- 

 tages of rapid absorption in the first instance, the darker a 

 man's skin the less is he afflicted by the retention of heat, which 

 would prevent a white man from working for any length of time 

 under a tropical sun, and the more suited is he to undertake the 

 required exertion. There may or may not be some uniformity 

 in the process by which it is acquired — there is certainly a 

 complete difference in the nature of the benefit derived — but 

 it is at least extremely probable that tropical men and arctic 

 insects do derive certain appreciable advantages from the dark- 

 ness of their colouring, under absolutely opposite conditions. 



* ' Savage Africa,' p. 18. 



t 'Smithsonian Report,' 1870, p. 387. 



Trans.Y.N.U., 1883 (pub. 1885). Series D 



