226 
Gradients of temperature, light, moisture, etc., 
were established by the earlier experimenters 
with a view to determine the conditions which 
the animals “preferred.” Others tested the 
reactions of animals in gradients of food and 
other chemicals in connection with investi- 
gation of senses and sense organs The earlier 
work was very general, but later the study of 
the reactions of protozoa in accurately deter- 
mined temperature gradients? was undertaken. 
In the study of reactions to light, Yerkes? 
first used the cylindrical lens and established 
definite and accurate light gradients. Mast* 
developed methods of measuring light. 
In 1911° the writer and Dr. Allee devised a 
gradient tank in which a definite permanent 
gradient of substances dissolved in water can 
be obtained. This was later improved and 
some of the sources of irregularity eliminated. 
A figure of the improved tank is shown by 
Wells. He tested the gradient fully with a 
conductivity cell and established the general 
character of the gradient for salts.7 The 
writer’ also devised a gradient cage for air 
work, air of different character being driven 
across the respective thirds of a cage so as to 
give three kinds of air with a slight mixing at 
the meeting points. 
In connection with the study of fishes the 
writer devised a method of graphing the move- 
ments of animals in gradients. This is illus- 
trated on the accompanying chart, graph 1. 
Here the distance from right to left represents 
the length of the gradient tank or the distance 
at right angles to the iso-lines such as iso- 
therms, that might be drawn across it. In 
making the graphs, movements along the iso- 
lines are ignored and only movements which 
change the position of the animal in the 
stimulus are represented. Thus in graph 1 
2 Davenport, Expt. Morphology, Vol. I., pp. 260- 
262. 
3 Yerkes, Mark Anniversary Volume, p. 361. 
4 Mast, ‘‘ Light and the Behavior of Organisms,’’ 
Philadelphia. 
5 Shelford & Allee, Jour. Expt. Zaol., Vol. XIV., 
p. 226. 
6 Wells, Biol. Bull., Vol. XXIX., p. 223. 
7 Wells, Jour. Exp. Zool., Vol. XIX., p. 246. 
8 Shelford, Biol. Bull., Vol. XXV., p. 81. 
SCIENCE 
[N. S. Vou. XLVIITI. No. 1235 
the distance from right to left represents the 
length of the cage. The scales at the sides 
represent time in minutes, each of the larger 
units being two minutes. The movements of 
the animal are drawn to the time and cage 
length scale. The animal shown in graph 1 
(a white-footed wood mouse) remained in the 
still air (left third) 45 seconds, spent fifteen 
seconds going the middle of the high velocity 
third (right third), turned back and returned 
twice, and was headed toward the still air 
end at the end of one minute and thirty sec- 
onds after the beginning, ete. These graphs 
can not be made with mechanical exactitude 
but a high degree of accuracy can be at- 
tained. With a watch adjacent to the cage 
and a little practise there is no difficulty in 
making such records. 
The gradient as encountered by the animal 
is usually a uniform increase or decrease in 
the intensity of stimulation except in the air 
work where there are three steps with two 
steep gradients between them. In all the ex- 
periments cited here one end of the container 
is near to the optimum for the species. In 
experiments where the optimum is near the 
center animals turn back from both ends. 
There is nothing to cause the animals to move. 
They do so spontaneously or because of the 
unusual surroundings. In the narrow tank 
they must go lengthwise; in so doing they en- 
counter the stimulus. No marked adaptation 
appears to take place. Goldfish were observed 
to behave in the manner described below for 
six hours. Controls in which the animal en- 
counters no change in going the length of the 
tank are not shown here. In the work cited 
controls accompany all experiments and have 
been considered in the ease of all work here 
discussed. A number of psychological points 
arising in this connection such as learning, 
Weber’s® law, etc. are discussed in the papers 
cited. The purpose of this account is to com- 
pare animals of different groups. 
In watching the behavior of mammals, 
reptiles, amphibians, fishes, insects, myriopods, 
annelids, flatworms and protozoa in gradients 
of temperature, light, moisture, air velocity, 
9 Powers, Biol. Bull., Vol. XXVI., pp. 177-200. 
