A considerable number of simple experiments of this kind were tried with results similar to those just given.  The frog apparently examines its surroundings carefully, and just when the observer thinks it has made itself familiar with the situation it reacts in such a way as to prove beyond doubt the absence of all adaptation.  In all these experiments it should be said, for the benefit of any who may be trying similar work, that only animals of exceptional activity were used.  Most green frogs when placed in the experiment box either sit still a great part of the time or jump about for only a short time.  It is very important for studies of this kind, both on account of time saving and the obtaining of satisfactory records, to have animals which are full of energy and eager to escape when in confinement.  By choosing such subjects one may pretty certainly avoid all unhealthy individuals, and this, it seems to me, counterbalances the disadvantage of taking animals which may be unusually quick in learning.

C. Complex Associations.

1. Labyrinth Habits.—­A more thorough investigation of the associative processes, sensory discrimination and the permanency of impressions has been made by the labyrinth method.  A wooden box, 72 cm. long, 28 cm. wide and 28 cm. deep, whose ground plan is represented by Fig. 1, served as the framework for a simple labyrinth.  At one end was a small covered box, A, from which the frog was allowed to enter the labyrinth.  This entrance passage was used in order that the animal might not be directed to either side by the disturbance caused by placing it in the box. E, the entrance, marks a point at which a choice of directions was necessary. P is a movable partition which could be used to close either the right or the left passage.  In the figure the right is closed, and in this case if the animal went to the right it had to turn back and take the left passage in order to get out of the box.  A series of interrupted electrical circuits, IC, covered the bottom of a portion of the labyrinth; by closing the key, K, the circuit could be made whenever a frog rested upon any two wires of the series.  When the frog happened to get into the wrong passage the key was closed and the animal stimulated.  This facilitated the experiment by forcing the animal to seek some other way of escape, and it also furnished material for an association.  Having passed through the first open passage, which for convenience we may know as the entrance passage, the animal had to choose again at the exit.  Here one of the passages was closed by a plate of glass (in the figure the left) and the other opened into a tank containing water.  The box was symmetrical and the two sides were in all respects the same except for the following variable conditions.  At the entrance the partition on one side changed the appearance, as it was a piece of board which cut off the light.