CONDITIONS OF EXISTENCE 



17 



light, and the presence of light retards the development of insects 

 normally living in darkness. 24 Experience with short wave lengths* 



*tcooJ-<r-~™p^c r >oo^-CT>'3-OLr)— '. «? 

 '"O'PorJior^ocviirir^cip'iinog g 



80 



75 



70 



65 



60 



55 



50 



45 



40 



35 



30 



25 



20 



°c 



26.7 



23.89 



21 11 



1833 



15.56 



12.78 



10.00 



7.22 



4.44 



1.67 



1.11 



3.89 

 6.67 



12 3 



5 6 7 8 9 10 11 12 13 14 15 16 17 



Fig. 1. — Graphs showing mean monthly rainfall and temperature for a tem- 

 perate climate (lower) and for a tropical climate with wet and dry seasons 

 (upper). Temperature is given on the vertical, and rainfall on the horizontal, 

 scale. 



for the human skin proves that too much light may be harmful to 

 animal tissues. Pigment layers at the surface of the skin afford effec- 

 tive protection against light. This seems to be the meaning of the 



* Knowledge concerning these physiologic effects is increasing rapidly (see 

 Janet Clark, Physiol. Rev., 1922, and Laurens, ibid., 1928), but the effect upon 

 geographic distribution awaits investigation (see L. Hill, Proc. Roy. Soc. London, 

 A, 1927, Vol. 116: 268-277). 



