228 
dissolved chemicals, lack of oxygen, ete., and 
in examining several hundred graphs made of 
their movements one is impressed with certain 
almost universal features of the reactions. 
In nearly all cases the animal becomes more 
sensitive to the stimulus after encountering 
its maximum intensity in the tank. It turns 
back from a weaker stimulus after encounter- 
ing a stronger one; the threshold of stimula- 
tion is lowered. In the case of some animals 
the increased irritability persists for an hour 
or more while in others it lasts only a few 
minutes. In the latter case a few turnings 
in the lower intensity or a brief stay in the 
lower intensity permits a return to the orig- 
inal sensibility and the animal enters the 
highest intensity again, a rhythm of increased 
and decreased irritability thus occurs. 
The graphs shown on the accompanying 
chart are selected to illustrate several stimuli 
and the reactions of several animal groups. 
Graph 12° shows the reaction of a white- 
footed wood mouse in a gradient of air veloc- 
ity producing evaporation as shown at the 
head of the graph in tenths cubic centimeters. 
Tt will be noted that after entering the high 
velocity air at the right once the subsequent 
movements in that direction were each shorter 
than the preceding. Graph 27° shows the re- 
action of a mouse to air of different humid- 
ities. The method of reaction is similar to 
that to moving air. The relative humidity in 
the thirds is shown at the end of the graph 
and the evaporation at the beginning. 
Graph 311 shows the reaction of a horned 
lizard to air. of high evaporating power, the 
humidity beine shown at the bottom and the 
evaporation in tenths cubic centimeters at 
the beginning. The invasions of the dry air 
during the first three minutes were followed 
by invasion only into the medium air. These 
are followed by six invasions of the dry air 
but with very short stays. Following this, 
from the thirteenth to sixteenth minute, the in- 
vasions reached only just through the medium 
air. The rest of the graph shows similar modi- 
fications and illustrates the rhythmic raising 
10 Chenoweth, Biol. Bull., Vol. XXXII., p. 192. 
11 Weese, Biol. Bull., Vol. XXXVIL., p. 115. 
’ SCIENCE’ 
[N. 8. Von, XLVIII. No. 1235 
and lowering of the threshold of stimulation. 
Graph 4 (from Weese—unpublished) shows 
the reaction of a horned lizard in a tempera- 
ture of substratum gradient; the temperature 
in centigrade at the ends and center is shown 
by the figures at the beginning of the graph. 
Each excursion in to the higher temperature 
is followed by shorter invasions reaching only 
to medium temperatures. 
Graph 5 shows the reaction of a common 
toad in a gradient of air moisture and evap- 
oration, the evaporation is given at the be- 
ginning of the graph and the humidity at the 
end. In this case each invasion of the dry air 
is shorter than the preceding ones. Graph 6 
shows the reaction of a sun fish (Lepomis 
humilis) in a gradient of ammonia in alkaline 
water. The fish was positive to the ammonia, 
and made excursions only into the low con- 
centration of ammonia, following each in- 
vasion into the water nearly free from am- 
monia. 
Graph 7 shows first graph of the reaction 
of a ground beetle taken at random, in light 
intensity gradient (Yerkes grader with Nernst 
lamp; triangular aperture). During the first 
five minutes the beetle avoided the stronger 
light in a lower intensity with each invasion, 
until it reversed and repeated the same rapid 
modification in invading the darker portions. 
After a five minute interval following the end 
of the first observation, the animal was trans- 
ferred to a grader in a horizontal position 
with an oblique cage, cylindical lens,!2 and 
triangular aperture. The oblique cage was 
turned so that in moving away from the source 
of light the animal came rapidly into lower 
intensities. Graph 8 shows the reaction of the 
same beetle in this gradient. Each invasion 
of the darkest part of the cage is followed by 
turning back a number of times in a com- 
paratively high light intensity. The rhythmic 
change in sensibility is well illustrated here 
also. 
Graph 9 (from Heimburger) shows the re- 
action of an earthworm (Helodrilus caligino- 
sus) in a gradient of air moisture The first 
twenty minutes which were spent in the dry 
12 Shelford, Biol. Bull., Vol. XXVI., p. 309. 
