ACTION OF FORCES ON ORGANISMS. 47 



not been so generally recognised. Professor Yung in 

 his series of highly ingenious experiments has reported 

 some very interesting and important facts in this 

 connection. He conducted with great care an ex- 

 periment in which a number of tadpoles were allowed 

 to develop under lights of different colours. None of 

 those in coloured light developed so well as those 

 which were in the natural light of day. There were 

 certain marked differences in the development under 

 the different lights. Thus tadpoles under a violet 

 light increased faster in size, but were not so active 

 as those under white or yellow light. Under violet 

 they grew better, but did not develop as well as 

 under yellow light. Professor Yung's observations 

 proved unmistakably that light may exert a potent 

 influence upon the developing organism. 



The most apparent phenomenon resulting from 

 the action of light on the animal kingdom is the de- 

 velopment of pigment. Certain transparent animals 

 being excepted, it is an almost universal rule that 

 animals exposed to light develop some pigment in 

 the skin. This pigment increases in quantity or 

 brilliancy as we approach the equator, where the light 

 is most intense. There is thus a direct ratio be- 

 tween the amount of light and amount of pigment. 

 This ratio appears also in another manner which is 

 so universal that it must have great significance. I 



