3io 
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- 
