The six individuals of Gasterosteus aculeatus tested learned to find their way into a small jar or bottle for food, and also learned how to come out. One failed in the standard test, with the jar upright. The solution was found at first by chance, but after a few trials the behaviour became definitely directed towards finding the solution. This change was accompanied by a sudden drop in the time taken; the jar also acquired significance, ceasing to be an indifferent object. The mode of solution was retained for over 3 months. The bearing of these results on the theory of learning is briefly discussed.

The flight reaction shown by sticklebacks on sudden illumination is described.

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