The animals used for these experiments were kept in the laboratory during the whole year in a small wooden tank. The bottom of this tank was covered with sand and small stones, and a few plants helped to purify the water. An inch or two of water sufficed; as it was not convenient to have a constant stream, it was changed at least every other day. There was no difficulty whatever in keeping the animals in excellent condition.
Of the protective instincts of the green frog which have come to my notice during these studies two are of special interest: The instinctive inhibition of movement under certain circumstances, and the guarding against attack or attempt to escape by ‘crouching’ and ‘puffing.’ In nature the frog ordinarily jumps as soon as a strange or startling object comes within its field of vision, but under certain conditions of excitement induced by strong stimuli it remains perfectly quiet, as do many animals which feign death, until forced to move. Whether this is a genuine instinctive reaction, or the result of a sort of hypnotic condition produced by strong stimuli, I am not prepared to say. The fact that the inhibition of movement is most frequently noticed after strong stimulation, would seem to indicate that it is due to the action of stimuli upon the nervous system.
What appears to be an instinctive mode of guarding against attack and escaping an enemy, is shown whenever the frog is touched about the head suddenly, and sometimes when strong stimuli are applied to other parts of the body. The animal presses its head to the ground as if trying to dive or dodge something, and inflates its body. This kind of action is supposed to be a method of guarding against the attack of snakes and other enemies which most frequently seize their prey from the front. It is obvious that by pressing its head to the ground the frog tends to prevent any animal from getting it into its mouth, and in the few instants’ delay thus gained it is able to jump. This is just the movement necessary for