On either side of the entrance there were grooves for holding card-boards of any desired color.  The letters R, R mark sides which in this case were covered with red; W, W mark white spaces.  These pieces of cardboard could easily be removed or shifted at any time.  At the exit the glass plate alone distinguished the sides, and it is not likely that the animals were able to see it clearly.  We have thus at the entrance widely differing appearances on the two sides, and at the exit similarity.  The opening from A into the large box was provided with a slide door so that the animal could be prevented from returning to A after entering the labyrinth.  The partitions and the triangular division at the entrance extended to the top of the box, 28 cm., so that the animals could not readily jump over them.

[Illustration:  FIG. 1.  Ground Plan of Labyrinth. A, small box opening into labyrinth; E, entrance of labyrinth; T, tank containing water; G, glass plate closing one passage of exit; P, partition closing one passage at entrance; IC, interrupted electrical circuit; C, cells; K, key in circuit; RR, red cardboard; WW, white cardboard.  Scale 1/12.]

The experiments were made in series of ten, with ten-minute intervals between trials.  In no case was more than one series a day taken, and wherever a day was missed the fact has been indicated in the tables.  The only motive of escape from the box depended upon was the animal’s desire to return to the water of the tank and to escape from confinement in the bright light of the room.  The tank was one in which the frogs had been kept for several months so that they were familiar with it, and it was as comfortable a habitat as could conveniently be arranged.  Usually the animals moved about almost constantly until they succeeded in getting out, but now and then one would remain inactive for long intervals; for this reason no record of the time taken for escape was kept.  On account of the great amount of time required by experiments of this kind I have been unable to repeat this series of experiments in toto on several animals in order to get averages, but what is described for a representative individual has been proved normal by test observations on other animals.  There are very large individual differences, and it may well be that the subject of the series of experiments herein described was above the average in ability to profit by experience.  But, however that may be, what is demonstrated for one normal frog is thereby proved a racial characteristic, although it may be far from the mean condition.