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 



