If it is not doubtful that the simulation is a phenomenon of differential sensitiveness, limited to cataleptic tetanus, and characterized by it, we must admit that its biological significance is still very obscure. Must we see in this phenomenon a special state, preserved by natural selection because it is advantageous to the species! This was for a long time the dominant opinion, some believing that the individual deceives its enemy by taking the attitude of a dead body, and others believing that the simple immobility deceives the enemy.
The first of these opinions was founded on vague appearances which do not support even a slightly attentive examination. Without doubt, many simulating species take a certain attitude, but this attitude is hardly that of a dead body. Darwin compares in this regard seventeen species of articulates (Julus, Cloportas, spiders, insects) and the result was "that in no case was the attitude exactly the same, and in certain ones the attitudes of the simulating individuals and the dead ones were as different as possible." Moreover, many insects take any attitude whatever at the moment of simulation; it suffices to take Ranatras from the water, says Holmes, "to put them into a state of immobility, which ordinarily lasts several minutes, and sometimes for more than an hour. The legs may be held against the body so that the insect resembles a stick, but they may also be stretched out at a right angle or bent in any direction whatever, this one one way and that one another, following the position in which they happen to be at the moment of simulation." One gets attitudes which are just as varied with the walking-stick of the laboratories, Dixippus morosus, which is not unlike the Ranatras in form. I have been able to convince myself that currant-worms do • not take a fixed position ; often they roll themselves in a ball like many other larvae, but often also they hold themselves straight, or bent, or regularly arched. This is not "simulation of death," and this term, unfortunately fixed by usage, gives a very inexact idea of the behavior of the animal.
But should we not regard this rigid immobility which characterizes simulating articulates as an artifice of protection? This is hardly probable, because this immobility is often produced by the enemy which cannot be duped in this way, and finds it advantageous in making its capture. It is possible that occasionally immobility plays a protecting role, says Rabaud, very justly,' ' but it cannot serve as a basis for a suggestion of natural selection, especially because the reflex of immobilization produces with certain animals, the Zygaenas, for example, only an effect which is too fugitive to give it any real advantage. Moreover, I have collected precise facts which show that immobility deceives predatory insects very little. If for certain arthropods, such as Glomeris, the reflex immobilization is profitable, for others it is rather harmful. . . . Moreover, the existence of the antagonistic reflex renders it liable to come into effect at the wrong time and do away with all the defensive value which immobility had given them. ' '
In fact, we find ourselves in the presence of a nervous phenomenon which seems to play no part in the conservation of the species, which presents no relation to zoological affinities, and which appears to be a simple specific exaggeration of differential sensitiveness.
However it may be, the fact that decapitation does not prevent immobility from being produced is a proof that this phenomenon, with the articulates at least, should not be considered as tributary to psychism. But the cataleptic state can be provoked with a certain number of higher vertebrates, and the young herdsmen know how to produce it easily in the goats and the poultry which they are watching. Fabre tried this with a turkey when he was studying the hypnosis of insects. But travelers and biologists report that several birds (the sea swallow, according to Holm, the wild goose of Siberia, according to Wrangl) have recourse to this subterfuge when they find themselves in danger, and the fox is said to use the same artifice. If we can establish by a series of experiments the veracity of these statements, we shall have to admit that with higher animals this simulation may be voluntary and serve to protect them, but this is not the case with insects, where everything seems to show that it is purely automatic.