338 bulletin: museum of comparative zoology. 



reflex; so that whether the chemical change acted upon the pigment 

 directly through chemotropism or indirectly through a reflex, the 

 amount of migration would nevertheless be proportional to the 

 chemical reaction. This interpretation is quite in keeping with the 

 known facts : — first, that at equal intensity blue-violet is vastly more 

 potent than red actinically (about 20,000 times according to a photo- 

 graphic test which I made upon a Seed's "Gilt Edge 27" plate — 

 true however only for the given emulsion and gi\'en intensity); 

 secondly, that blue-violet is more efficient than red in CAoking the 

 migration of pigment. 



The final explanation of the pigment as a protective mechanism 

 would be, according to the above deductions, that it is correlated with 

 the sensiti\'ity of the receptive organs to those wave-lengths which 

 stimulate them to the greatest chemical activity. Since the data 

 from which the deductions have been made referred partly to the 

 compound eye and partly to the vertebrate retina, and since the 

 quantitative statement for the migration of pigment in both cases 

 was not very large in amount, and since, further, the facts were often 

 conflicting, the above conclusion is but a tentative one. 



