608 
PHYSIOLOGY: W. E. CARREY 
suits explain why a positively helio tropic animal with one eye black- 
ened approaches a Hght by a series of alternating small and large circles, 
the former being executed when the good eye is illuminated from the 
source of light, the larger when it is in the shadow. 
8. Differential sensibility. Robber flies with one eye blackened show 
the postural conditions in the most pronounced way in the early morn- 
ing or after being kept for some hours in the dark. Constant exposure 
to the light produces considerable fatigue of the eye with recovery in the 
dark. These facts among others suggested the possibility of producing 
a different sensitiveness of the two eyes and corresponding differences 
in the muscle tonus with asymmetry of position, and in physiological 
action of the muscles of the two sides of the body when the two eyes 
were equally illuminated. Such an experiment constitutes a crucial 
test of the tonus theory of heliotropism. It succeeded beyond our 
greatest expectations. Asphalt black was applied to the right eye of 
several specimens of Proctacanthus. In two or three days the paint 
had formed a brittle shell. During this time the blackened eye had 
become 'dark adapted.' When such a fiy is exposed to light, it tilts and 
circles to the left. If now the brittle shell is cracked off the right eye 
by carefully pinching with fine forceps, the exposure of this very sensi- 
tive eye to light results in a reversal of the whole picture ; the fiy circles 
toward the side from which the black was removed. Although the 
illumination of the two eyes is of equal intensity, what was the normal 
eye now becomes relatively a darkened eye owing to its lesser sensi- 
tiveness. A differential effect results, probably due to a difference in 
the rate of photochemical change in the two eyes. This reversal of the 
muscle tonus, and of forced motions, may persist for an hour or two or 
even longer, until the two eyes become, as they ultimately do, of equal 
sensitiveness and the fly behaves like a normal animal. 
These experiments are not only incompatible with any avoidance 
idea, for after removal of the black there is nothing to avoid, but they 
are also incompatible with the conception of 'habit formation,' for 
''habit" in the performance of the circling movements is of no avail 
when Hght is admitted to the darkened eye. The animals circle to 
that side because the tonus of the muscles is such that they are forced 
to do so. 
9. Post mortem rigor. Proctacanthi kept in a moist atmosphere but 
without animal food, have lived two or three weeks. During this time 
if one eye was blackened the tonus change in the muscles became almost 
a fixed condition, probably the result of an atrophy of the muscles or a 
lack of tonus, similar to the effects of disuse. Death supervening, the 
