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 given intensity); 
secondly, that blue-violet is more efficient than red in evoking 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 sensitivity 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. 
