424 The American Naturalist. ae 
EXPLANATION OF TABLE I. 
The areas marked I.-IX. were vertical parallelograms of equal size (34 
by 125 mm.), extending from the bottom to the surface, consecutively 
placed on the side of a large square aquarium, which was placed at a dis- 
tance of fourteen feet from a window three feet wide and eight feet high, 
facing the northeast, so that direct sunlight never fell upon the aquarium. 
The top of the aquarium was covered, the ends and rear side uncovered, so 
as to admit the diffused light of the room. Area IX. extended to within 
nine mm. of one end of the aquarium. I. was nearly in the middle. The 
Hydras had lived for about two months in the aquarium, and were very 
large and vigorous, many of them actively budding. Throughout the 
experiment there was a moderate supply of crustacean food, but the ani- 
mals nevertheless often descended to the bottom and filled themselves with 
sediment. The alternate areas II., IV., VI., VIII., were first covered with 
double layers of colored glass (for the color-test see page 421), as in the 
table, and these were allowed to remain for five days. The results were as 
follows: The total number of Hydras increased from 153 to 215,—#. ê., 40 
percent. The record of the colored areas (taking the mean of the first two 
and the last two observations) was: 
. Yellow ... . . decrease (percent)... .++ 56 
Red Ke a m E A 
Green . 5 eas ae Sa ae ae 7° 
Hoe So ks eae n ge 
The record of the light areas was : 
Ps +++ +. «increase cent.) <0 o tee 
ie oy ee on, Iert 
LA eee i “ “ . 89 
ooo “ u . 80 
Cee “ “ ek eee 
Thus all of the colors except the blue show a large dec ERT 
and all of the light areas a large increase. The increase in the blue is more — 
than double the general rate of increase, but less than that of the two 
areas, I. and IX. The colors are now rearranged, one layer of blue bees 
substituted for the green, two layers of blue for the red, three layers ee 
for the yellow, and four layers of blue for the former two layers of me bet 
Results, after four days, as follows (taking, as before, the mean a 
two observations) : 
