fail to see the method exemplified. If he were watching a chick pecking at a variety of objects and giving signs of disgust when it had seized a nauseous substance he would doubtless regard the process as one of trial and error whatever name he might apply to it. A study of the conduct of much lower organisms would disclose many cases almost equally evident. The lives of most insects, crustaceans, worms, and hosts of lower invertebrate forms including even the protozoa show an amount of busy exploration that in many cases far exceeds that made by any higher animal. Throughout the animal kingdom there is obedience to the Pauline injunction, "Prove all things; hold fast that which is good."

The trial and error method is set off by JENNINGS in sharp contrast to the usual scheme of tropic reaction. The tropism," he says, "leads nowhere; it is a fixed, final thing, like a crystal." And elsewhere: "This method of trial and error, which forms the most essential feature of the behavior of these lower organisms, is in complete contrast with the tropism schema, which has long been supposed to express the essential characteristics of their behavior. The tropism was conceived as a fixed way of acting, forced upon the organism by the direct action of external agents upon its motor organs. There was no trial of the conditions; no indication of anything like what we call choice in the higher organism; the behavior was stereotyped." (p. 250). If the term trial and error is used in the wide sense here employed I cannot but think that the distinction between the method so designated and the orthodox scheme of tropisms is not, after all, so wide as it at first appears. The motor reaction of Paramecium is certainly a fixed way of acting brought about almost inevitably by certain factors of the environment. Its behavior is certainly as stereotyped as that of any organism whose reactions are definitely known. Its reactions are "forced movements" in the untechnical sense of this expression, and there is no more evidence of choice in its conduct than in the contraction of a muscle; for we can scarcely speak of choice in a creature that reacts in one way to all sorts of stimuli. On the other hand the trial and error method may be ex

These devia

tended to cover the reactions of an animal which orients itself according to the usual scheme. If an earthworm when illuminated more on one side of the body than another simply turned away, by a direct reflex, from the greater stimulus and kept on doing so until the body was brought parallel with the rays we would have a case of orientation according to the commonly accepted theory of tropisms. The creature is forced into line by unequal stimulation of the two sides of its body. When in crawling one side of the body comes to be presented to the light more than the other deviation is corrected by a direct movement away from the stimulus. Getting out of line may be regarded as an error which brings about a certain reflex that sets the animal again upon a straight course. The worm keeps in an approximately straight path because those movements (errors) which bring it out of a straight course are counteracted by a definite reflex, while those movements (successful trials) which bring the creature away from the light are followed up. It is because these errors are corrected that the orientation even according to the ordinary theory, is maintained. tions are as much errors as the random movements towards the light in the method of orientation that is actually followed. The stimuli in ordinary tropisms may, however, serve to prevent errors as well as to correct them after they are made. Take an organism that orients itself by the direct method. As it swings out of line it is doing so against the influence of the unilateral stimulus which tends to turn it back during, as well as after, the trial. But the mechanism of preventing the movement and of reversing after it is made is, in this case, the same; only in proportion as the checking predominates does the random character of the movements becomes reduced. Between the behavior of an organism like an earthworm that jerks back and turns to one side when the anterior end is stimulated and that of one which turns directly when the side is presented to the stimulus there is to be sure a maked difference in behavior, but there is an underlying basis of similarity in the two cases in that in both errors are made, although they are corrected in different ways. The end result of both methods is the same, i. e.

to get the organism away from the stimulus. In the one case it is accomplished by a direct reflex, without more ado; in the other only after a considerable waste of energy in inconsequential vermiculations. In orientation according to the usual theory of tropisms errors are made in abundance; but they are corrected in a more direct and efficient way than in the more or less haphazard method so frequently followed.

By a careful analysis of the phototaxis of Stentor, Euglena, and some other protozoans JENNINGS has concluced that the oritation of these forms to light takes place according to the trial and error method, and not by the method of simple forced reflexes. The reactions of Euglena are of especial interest since this form apparently shows a combination of both direct and indirect methods of orientation to the direction of the rays. Euglena may react to a strong or sudden stimulus from the light by backing off and starting ahead in a new direction. Several trials of this kind may be made until finally the creature becomes oriented when it swims to or from the light according to the intensity of the stimulus. Euglena is also capable of orienting itself by gradually bending its course until it comes to be approximately parallel with the rays. Ordinarily this form swims in a straight spiral path. Should light shine on the body from one side the sensitive anterior end would be stimulated differently in different parts of its spiral course. According to JENNINGS, it is the diminution of light as the animal turns the anterior end away from the stimulus that causes the motor response. When the Euglena turns so that the anterior end is less illuminated, it is stimulated to swerve back further towards the light, and, by a succession of such responses, it finally becomes oriented to the direction of the rays. Swimming through that portion of its spiral course that causes the diminution of light at the anterior end is that part of the creatures activities that must be looked upon as error, if we go so far as to regard the passing through different sections of a continuous spiral course as trials. But to view the matter in this way is to go far towards obliterating the distinction between orientation through trial and error and orientation by the direct method. In the mode of photo

tactic response here considered Euglena does not react by a number of indiscriminate movements until the right one is accidentally hit upon, but by a direct reflex whose effect is to bring the organism more nearly parallel to the direction of the rays. The phototaxis of Euglena is not so manifestly the outcome of the trial and error method as that of the earthworm. In the latter case light does not cause directly a movement which makes for orientation. The direct response may or may not have that effect. The successful movement is accidentally hit upon, but one can scarcely say this in the case of Euglena in which the orientation takes place more nearly in accordance with the usual scheme.

It is perhaps difficult to decide where best to draw the line as regards the employment of the expression trial and error. If it is extended to include the phototaxis of Euglena and other protozoa where there is a gradual adjustment of the path by appropriate direct responses until it coincides with the direction. of the rays, we can hardly stop short of including, at least to a considerable degree, the cases of phototaxis that take place according to the commonly accepted theory. We may regard all departures from the straight and narrow path as errors according to whatever theory of phototaxis we may choose to adopt, and we can look upon all movements in that path as successful experiments. I would suggest that if the term trial and error is widened, as seems desirable, so as to include such reactions as are described in the first part of this paper where there is no discernible element of learning involved, its application be limited to those cases in which the adapted movements may be regarded as chance successes. This would exclude the tropisms of the orthodox kind; it would exclude the gradual orientation of such forms as Euglena where oblique stimulation causes a direct response which brings the body more nearly parallel to the rays. It would include many of the reactions of the protozoal where, as in the phototaxis of the blue Stentor, the right direction of movement is hit upon by chance, and a large part of the actions of higher forms. All organisms make errors. In some cases these errors are rectified by an appropriate direct reflex,

in others by the chance success of a random movement. There will doubtless occur many cases difficult to classify where trials are not perfectly random movements but where the stimulus may have a certain directive effect which is in large measure obscured. A tropism of the direct sort is not necessarily a perfectly fixed and rigid affair. It may be a tendency more or les obscured by a lot of random movements arising from internal causes. An organism may be drawn to a certain point through a direct orienting reflex, but if there be at the same time a large element of random activity in its behavior it may seem to reach that point by the method of trial and error. In the trial and error method the random character of the movements impresses us most; in the tropisms, the element of direct determination by the environment. Both of these factors run

through the behavior of all animals, but they are mingled in various proportions in different forms. In the lives of most, if not all, animals both are essential elements in the adjustment of the organism to its conditions of existence.

University of Michigan, Ann Arbor, Mich., Dec. 6, 1904.