XV 
THE RELATION OF FACTS TO THEORIES 
313 
relation between the molecular weight of these pig¬ 
ments and their respective colours, and that this 
gradual development of colour in the history of the 
individual corresponds to the evolution of colour in 
the history of the race. 
We must now return to a more detailed consider¬ 
ation of Simroth’s paper. He supports his central 
thesis as to the origin of all pigments showing simple 
spectral colours from a prime substance by three 
arguments, which are not, however, very sharply 
differentiated from one another. 
His first argument is based upon the modifica¬ 
tions of the retinal purple in Vertebrates. As is well 
known, the rods of the retina of most Vertebrates 
contain a purple pigment known as rhodopsin or 
“ sehpurpur,” which, when exposed to light, under¬ 
goes a series of changes—becoming red, orange, 
yellow, and finally colourless. These modifications 
Simroth, so far as the author understands him, 
regards as evidence that all pigments are genetically 
related, and that one can be derived from another. 
He also lays especial stress upon the fact that red 
pigment is usually associated with the eye-spots of 
simple organisms, and that such organisms seem 
never to possess dark-coloured pigments. This he 
regards as evidence that pigments belonging to the 
less refrangible end of the spectrum tend to appear 
first. 
The second argument is based upon the modi¬ 
fications of the lipochromes of plants. Simroth 
regards chlorophyll as the result of the modification 
of a lipochrome, a view for which, as we have seen, 
there is practically no evidence. He also believes 
