322 ANIMAL COMMUNITIES 



are fundamental in ecology. We can sketch out here only such methods 

 as are modifications of the usual method of these branches of biological 

 science in such a way as to be intelligible to those somewhat familiar 

 with such laboratory methods. 



(a) Experiments in the field are of prime importance in ecological 

 work. Here smaller animals can be secured in numbers and subjected to 

 experimental conditions before their physiological state has been modi- 

 fied by bad treatment. Any student competent to undertake ecological 

 investigation will find no difficulty in devising apparatus which can 

 be carried into the field and which will enable him to do work of a 

 high degree of scientific accuracy. Each experiment should be accom- 

 panied by a control. That is, the same number of animals should be 

 put under the same conditions as in the experiment, except for the one 

 factor which is to be varied. For example, in an experiment designed 

 to determine the reaction of animals to light, the control should be 

 either equally lighted or entirely dark (more easily accomplished), and 

 the experiment which is exactly the same except that the light ranges 

 from darkness to bright sunlight. 



The apparatus which we have just begun to develop for this purpose 

 is still in need of much perfecting. Thus far it consists of granite-iron 

 and galvanized-iron containers about 13 in. long, 3 in. deep, and 4 in. 

 wide. These are provided with galvanized-iron covers, somewhat larger, 

 and a little deeper. One of these is provided at one end, with an adjust- 

 able slide which may be used to open a slit to admit light when desired. 

 In connection with this slit a mirror is provided with which the sunlight 

 may be projected into the pan as nearly vertically as possible. The rays 

 are allowed to pass through a water screen to cut out the heat. For work 

 with temperature the same receptacles have been used and temperature 

 differences secured by placing one end of the experimental tank in contact 

 with hot soil and the other with cold soil. Land animals are confined in 

 tubes 11 in. long by if in. in diameter with round bottom and close- 

 fitting cap, shaped like the bottom. Reactions to gravitation have been 

 tested with the use of wire cylinders for land animals, and glass cylinders 

 lined with screen for aquatic animals. Black covers are used to exclude 

 light in various ways as a check. For the study of reactions to current 

 two long galvanized boxes (24X5X4 in.) have been used, one having 

 screen ends and the other tight ends. They are placed in the stream 

 side by side, one serving as an experiment, the other as a control. Large 

 tin pans have been used in connection with the long boxes, the water in 

 the experiment being stirred so as to produce a circular current, while the 



