ZOOLOGY: CROZIER AND AREY 
497 
important in determining the general habits of the animal. Through the 
mediation of the nervous elements of the shell-valves Chiton exhibits 
precise photo tropic orientations. The younger individuals are photo- 
negative, the older ones photopositive, to sunHght. Chitons of inter- 
mediate age are positive to weak daylight, negative to strong. The 
'intermediate' age is generally in the neighborhood of six years (5 to 7 
cms. length). 2 This alteration in the behavior of Chiton toward day- 
light as the animal becomes older is primarily responsible for the exhibi- 
tion of a complex series of harmonious environmental interrelations. 
The progressive inversion in the sense of Chiton's phototropism is due to 
the elimination of the shell photoreceptors by erosion, thus conditioning 
in the older individuals a lower specific stimulating power of the light. 
Erosion of the shell is itself produced through normal growth effects, in 
part; more directly, by physical agencies of the environment and by the 
activities of various small organisms which come to live on the chiton 
shell. 
The nature of the algal food, varying somewhat from place to place, 
and the presence of epiphytes, barnacles, tubiculous worms, and the like, 
on the dorsal surface of a chiton, determine automatically a certain 
degree of homochromic coloration. The established fact that a given 
Chiton tends to remain within a relatively small area for long periods, 
affords opportunity for the institution of correlations of this nature. 
These correlations concern practically every feature of the bionomics of 
Chiton: its method of feeding, its breeding habits, and other matters 
which have been studied; and they follow automatically in the wake of 
the changing phototropism of the animal, itself largely controlled by en- 
vironmental causes. Granted the original condition that those inherited 
mechanisms for response present in the young post-larval chiton lead to 
its Kving under loosely piled small stones at the very upper limit of the 
tide, it can be shown that the 'adaptive' cycle of subsequent changes, 
leading ultimately to the life of the oldest individuals on brilHantly 
illuminated rock surfaces, is produced in automatic sequence. It is 
important that the environmental effects of a particular location are in 
general not such as to cause a chiton to become more closely adapted to 
the peculiarities of that situation, but on the contrary result in the ani- 
mal's going somewhere else — namely, into a more brightly illuminated 
area, where the environment as a whole is in certain respects fundamen- 
tally different. 
From the standpoint of the theory of adaptation, the most important 
result of our inquiry into the natural history of Chiton may be given thus: 
