It may be said, of tropisms in general, as we have said of phototropism in particular, that they are automatic and, so far as they distinguish sensations, independent of any choice and consequently of all psychic phenomena. The responses of the organism to the external stimuli are exactly determined by the nature and intensity of the excitant.

Stereotropism, especially a certain kind of stereotropism, manifests itself under the form of an orientation in which the great axis of the symmetrical organism is brought into the direction of the stimulating forces, so that the subject, which is incapable of deviation, proceeds toward the stimulating source or the reverse, according as the tropism is positive or negative. It receives, then, from each side and at the same angle the influence of the stimulus. When this is represented by two distinct sources, the organism oscillates to an intermediate position where both sides are equally excited. This last character, placed in evidence by Bohn, is a criterium of tropisms which distinguishes them from psychic acts, properly so called, where the faculty of choice comes in. In many cases, and notably with the articulates, where the stimulus is received in the cephalic region of the body (eyes, antennae) another character of tropisms is furnished by the functional suppression of one of the two symmetrical receptive centers. Then the stimulus acts only on one side and the animal turns after manoeuvering.

Such, according to observation and experiment, is the objective side of these phenomena. To know the mechanism, to know how they are released by the excitation of external forces, it is necessary to study the constitution of the organisms. These, says Loeb, show usually a symmetrical structure, not only morphologically, but chemically and dynamically. By this I mean that the symmetrical parts of the body possess the same chemical substances, qualitatively and quantitatively, so that the rapidity and the nature of the chemical reactions are the same in the symmetrical parts. On the other hand, the asymmetrical parts of the organism have an entirely different chemical composition, so that the nature and the rapidity of their reactions are not necessarily the same, and, generally speaking, are not the same.

One may suppose-and it is true, in all probability- that the forms of energy which influence the orientation, or the direction of the movements of an organism, act as they do because they alter the rapidity or the character of the reaction.

"Because of this symmetrical structure," he says,i1 organisms orient themselves automatically in the whole field of forces that affect their chemical reactions ... in such way that the symmetrical parts of the body surface meet the direction of the forces at the same angle" and receive the forces in the same degree. If the body turns aside in a certain direction, it receives more force on the other side, which thenceforth brings a greater abundance of provocative movement reactions; the lost orientation is immediately recovered.

This physicochemical explanation of tropisms rests, as Loeb says, on an hypothesis, but "a probably true hypothesis," for tropisms are only the responses of living matter to the stimulations of external forces, and we know that protoplasm has a very great physicochemical sensitiveness. Our detailed study of phototropism, moreover, shows that this hypothesis is fully justified by the facts.

We are thus led to the origin of tropisms, a field open to research, which must be explored. It is the part of science to reach as far as possible toward the first cause of phenomena; but here, as always, it is the first cause that escapes us. Tropisms are essentially characterized by directive movements and it is necessary to know why a chemical phenomenon, provoked by external forces, has, as a corollary, a motive reaction,-in fact, a determined motive reaction,-because certain organisms appear insensible to these forces; also, why certain ones respond in one way to the stimuli and others in a way diametrically opposite. We are here in the presence of the enigma of living matter and if we deny that life manifests itself by an ensemble of physicochemical phenomena, we should recognize that the continuity of these phenomena and their coordination in their variability escape us completely. But this continuity and this coordination seem, indeed, to be the very essence of life. Living matter is a protean thing in perpetual evolution and constant change. One can grasp it, but not fix it. Piéron says :

If to-morrow, the chemists should produce a true protoplasm, do we know that this protoplasm would function and give us even a fugitive view of life, a passing glimpse? Would it continue to live, really live?- for continuity is one of the principal characteristics of life. It could, if it possess this property, of which we are without doubt far away from the knowledge of the conditional formula, but which appears to us to be really the foundation even of psychism.1

Are tropisms adapted? As soon as they issue from the egg in the spring the larvae of the Brown-tail Moth, urged by a powerful positive phototro-pism, climb the twigs and find there the young leaves necessary for their subsistence. The tro-pism which affects them responds fully to their immediate needs. It is the same with the negative phototropism which urges fly larvae into the midst of the material in which they find nourishment, with ithe positive chemotaxy which leads male insects after their females, with the hydrotropism which directs water-beetles and water-bugs, from far away, toward water. These are the tropisms which can qualify as useful, because they respond exactly to the needs of the species. Many others have the same character and they, in fact, appear to be in the majority.

1 H. Piéron, L'Evolution du psychisme {Revue du mois, 1908, v. 5. pp. 291-310).

However, there are tropisms that appear useless. What is the use, says Loeb, of the positive heliotropism of the mining larvae of Cossus ligni-perda which lives in darkness, and that of Cuma rathhei which burrows in the depth of sea ooze? A profound biological study of these creatures will perhaps make possible a reply to this question. But, according to Loeb and Maxwell, the shrimps of the genus Palœmonetes are very sensitive to eletricity. They orient themselves in the direction of the currents which traverse their aquarium and head toward the anode. But this galvanotropism cannot be an adaptation. " Galvanotropismi ' says Loeb, "is purely a laboratory phenomenon. Outside the laboratory no animal would find itself in a situation demanding a gal-vanotropic response; nevertheless, galvanotropism is not rare with animals. " If it is objected that mineral substances orient themselves in the same way under the influence of electrical currents, it will be easy to cite numerous animals which react by chemotropism to chemical agents absolutely foreign to the environment in which they live and to which it has been, therefore, impossible to adapt themselves.

Finally, there are tropisms which are sometimes injurious, since they lead insects to their destruction. This is the case, for example, with the positive phototropism which every night leads thousands of insects into the flame. But fire is not •an invention of man. Before human fire there was that of incandescent lava; and volcanic fire, from which man undoubtedly got his first flame, goes back to the origin of the earth. In spite of everything, insects continue to fly to lights, and should they perish, they are obeying the innate impulse of their positive phototropism.

If useless and destructive tropisms are not the result of adaptation, can we believe that the others have been able to modify themselves by a kind of physiological selection, by an active adaptation of the organism to their influences? This question is vigorously debated. Loeb and his disciples reply in the negative; but Jennings considers the varied movements of an animal in tropic reaction as a series of trials, correcting, little by little, previous errors. Jennings 's theory will be discussed in Chapter IV, but here and now we wish to say that it supposes an adaptation to chemical phenomena of which the living matter is the seat. This way of looking at it has recently been sustained by Pieron1 in a hypothetical form. This biologist observes:

It is not absurd to think that chemical properties are such, in living protoplasm, that a negative reaction in its simplest form-a contraction, a retreat-tends to develop a factor effective in dimishing the vitality of the protoplasm, that is to say, reducing its changes, modifying, in the spnse of diminishing, its chemical equilibrium.

The protoplasm, in effect, accustoms itself to the influences of external factors. Davenport has given numerous examples and no one denies that man can inoculate himself against poisons. But, between this acquired non-sensitiveness, really temporary, and the adaptive modifications of tropic reactions is a gap,-I do not wish to say an abyss. I do not wish to call it an abyss because Pieron's proposition rests in the domain of possibilities although its author considers it "quite hypothetical." If one leaves the domain of hypothesis to enter that of facts, one may state that tropisms vary with the physiological state of the organism, or of parts of the organism, and that they invariably follow the modifications of this state without being the cause that provokes it. The adaptive selection of tropic reactions appears, indeed, to be absolutely null.

1 H, Pieron, Log. cit.

But insects which respond to tropic reactions are subject to natural selection. This does not give birth to tropisms 'any more than to other characters; it simply eliminates all the forms in which these phenomena, as Loeb says, "are incompatible with reproduction and conservation." And it is thus that the real utility of the very great majority of tropisms is explained. Without doubt certain disadvantageous tropisms have persisted, but it appears that these tropisms are useful in a number of cases, for they do not place the species in peril. The noctuids, the May-beetles, the flies, and thousands of other insects which burn themselves in natural or artificial flame have an unusual power of multiplication; this positive phototropism can have no effect on that and it is simply a superabundance which is limited to the great advantage of the natural harmonies.