92 GIBBS. 



not only by latitude and longitude, but also by altitude and local 

 meteorologic conditions, the last varying from season to season, 

 from day to day, and during different hours of the day. 



Excluding these conditions, I do not believe that, when the 

 normal intensities are compared, the light of the Tropics is dif- 

 ferent from the sunlight of any other region. This statement is, 

 perhaps, more revolutionary than it seems at first thought for 

 it conveys the idea that manj^ of the conditions which have 

 been treated as being peculiar to the Tropics and many of the 

 effects attributed to tropical sunlight can be reproduced in 

 regions outside of the Tropics. This I know to be true in some 

 cases, and no doubt it will be proved for others upon due experi- 

 mentation. 



Nevertheless, the effect of sunlight per se upon life in the 

 Tropics can not be ignored to the same extent that it can be 

 in the Temperate and Arctic Zones even though the light values 

 are taken as equal, or with differences too small to be of great 

 account ; for, in the Tropics, other factors increase the ill effects 

 of light so that the organism is not so able to resist. In other 

 words, during days of equal length the individual in the Tropics 

 is not only affected by the sunlight practically to the same degree 

 as his brother in the Temperate Zones, but in addition he is 

 subject to unfavorable or disagreeable conditions of temperature; 

 the factor of monotony, due to the small variation in the daily 

 duration of sunshine; and other things which influence health. 



Peculiar conditions which have often been attributed by many 

 writers to tropical sunlight I believe to be due, not to the fact 

 that the character of the light is different from that of the 

 temperate regions, but to other meteorologic modifications which 

 go to make up climate; namely, duration of sunshine, clouds, 

 rainfall, winds, and humidity, all of which affect the air tem- 

 perature; the last is probably the most important factor and 

 depends to a large extent upon the duration of sunshine. 



It has been realized from earliest times that different races 

 are more or less adapted to certain different climates.^ How 

 the changes in racial characteristics and the changes in the 

 individual which produce acclimatization have been brought 

 about are not well understood. It has long been believed that 



"Woodruff has considered the question involved in his works: The 

 Effects of Tropical Light on White Men. New York (1905) ; and Expan- 

 sion of Races. New York (1909). 



