310 COLOUR IN NATURE chap. 



summary, yet it may quite well be that the subjective 

 element is far from being absent. 



So far, then, as I understand Dr. Simroth, he 

 refers all pigments back to a prime substance which 

 is closely united to primitive protoplasm, and which 

 has evolved along with primitive protoplasm, pro- 

 ducing the simple spectral colours in the order of 

 the spectrum, beginning at the red end. That is, 

 red pigments are simpler in composition than those 

 of green or violet colour, and tend to appear earlier, 

 and to be particularly prominent in simple organ- 

 isms. We may thus speak of an evolution of pig- 

 ments corresponding to an evolution of organisms, 

 and the red or yellow pigments correspond to the 

 simpler organisms. These red and yellow pigments 

 have a simple chemical composition and a small 

 molecular weight, and as we pass upwards and find 

 the colours of the pigments changing, so also we find 

 the chemical composition growing more complex and 

 the molecular weight increasing in amount. 



As to the causation of this evolution of pigment. 

 Dr. Simroth refers primarily to the effects of light 

 and warmth, but makes the following detailed sug- 

 gestion as to the determining cause of the actual 

 direction of evolution. 



In the first place, he suggests that at an early 

 stage in the world's history the atmosphere was so 

 saturated with watery vapour, that it at first only 

 allowed the red rays of the sun's light to pass through, 

 and then, as the vapour gradually cleared away, the 

 other rays, in the direction from the red to the violet, 

 were able to penetrate. 



Secondly, he believes that protoplasm is so con- 



