Of the different kinds of adaptation none are more remarkable than those connected with the immediate responses of organisms to external agents. These responses are usually thought of as associated with the nervous system; and while in the higher forms the nervous system plays an important role in the reaction, yet in many cases it is little more than the shortest path between the point stimulated and the muscles that contract; and in the lower animals, where we find just as definite responses, there may be no distinct nervous system, as in the protozoa, for instance.

Many of the so-called instincts of animals have been shown in recent years to be little more than direct responses to external agents. Many of these instincts are for the good of the individual, and must be looked upon as adaptations. For example: if a frog is placed in a jar of water, and the temperature of the water lowered, the frog will remain at the top until the water reaches 8 degrees C., when it will dive down to the bottom of the jar; and, if the temperature is further lowered, it will remain there until the water becomes warmer again, when it will come to the surface again. It is clear that, under the ordinary conditions of life of the frog, this reaction is useful to it, since it leads the animal to go to the bottom of the pond on the approach of cold weather, and thus to avoid being frozen at the surface.

Another illustration of an instinct that is a simple response to light is shown by the earthworm. During the day the worm remains in its burrow, but on dark nights it comes out of its hole, and lies stretched out on the surface of the ground. It procures its food at this time, and the union of the individuals takes place. In the early morning the worm retires into its burrow.

This habit of the earthworm is the direct result of its reaction to light. It crawls away from ordinary light as bright as that of diffuse daylight, and, indeed, from light very much fainter than that of daylight. If, however, the light be decreased to a certain point, the worm will then turn and crawl toward the source of light. This lower limit has been found by Adams to be about that of .001 candle-metre. This corresponds to the amount of light of a dark night, and gives an explanation of why the worm leaves its burrow only at night, and also why it crawls back on the approach of dawn. It is also obvious that this response is useful to the animal, for if it left the burrow during the day, it would quickly fall a prey to birds.

The blow-fly lays its eggs on decaying meat, on which the larvÆ feed. The fly is drawn to the meat by its sense of smell, a simple and direct response to a chemical compound given off by the meat. The maggot that lives in the decaying meat is also attracted by the same odor, as Loeb has shown, and will not leave the meat, or even a spot on a piece of glass that has been smeared with the juice of the meat, so long as the odor remains. Here again the life of the race depends on the proper response to an external agent, and the case is all the more interesting, since the response of the fly to the meat is of no immediate use to the fly itself, but to the maggot that hatches from the egg of the fly.

The movement toward or from a stimulating agent is, in some cases, brought about in the following way. Suppose an earthworm is lying in complete darkness, and light be thrown upon it from one side. The worm turns its head, as it thrusts it forward, to the side away from the light; and as it again moves forward, it continues to bend its head away from the light, until it is crawling directly away from the source. When the light first strikes the worm, the two sides will be differently illuminated. This causes a bending of the head, as it stretches forward, toward the side of less illumination, and the bending is due to a stronger contraction of some of the muscles on the less illuminated side; at least the reaction appears to be due to a simple response of this kind. When the body has been so far turned that the two sides are equally illuminated, the muscles of the two sides will contract equally, and the movement will be straight forward and away from the light. If the reaction is as simple as this (which is in principle the explanation advanced by Loeb), the result is a simple reflex act, and need not involve any consciousness or intentional action on the part of the worm to crawl away from the light. In fact, the same reaction takes place when the brain is removed, not so quickly or definitely, it is true, but this may be due to the removal of the anterior segments of the worm, in which part the skin appears to be more sensitive to light than elsewhere.

Another factor that plays an important rÔle in the habits of the earthworm is the response to contact,—the so-called stereotropism. If, in crawling over a flat surface, the worm comes in contact with a crevice, it will crawl along it, and refuse to leave until the end is reached. The contact holds the worm as strongly as though it were actually pulled into the crevice. It can be forced to leave a crevice only by strong sunlight, and then it does not do so at once. If the worm crawls into a small glass tube, it is also held there by its response to contact, and the smaller the tube, the more difficult is it to make the worm leave by throwing strong sunlight upon it.

Loeb has found that when winged aphids, the sexual forms, are collected in a tube, and the tube is kept in a room, the aphids crawl toward the light. This happens in ordinary diffuse light, as well as in lamplight. It is stated that the animals orientate themselves towards the light more quickly when it is strong than when it is weak. They turn their bodies toward the light, and then move forward in the direction from which the rays come. It can be shown by a simple experiment that the aphids are turned by the direction of the light, and not by its intensity. If they are placed in a tube, and the tube laid obliquely before a window in such a way that the direct sunlight falls only on the inner end of the tube, the aphids will, if started at the inner end of the tube, first crawl toward the outer surface of the tube, and then wander along this wall, passing out of the region of sunlight into the end of the tube nearest the window, where they come to rest at the end. They have moved constantly towards the direction from which the rays come, passing, as it were, from ray to ray, but each time toward a ray nearer the source of the light.

If the tube be turned toward the window, and the window end be covered with blue glass, the aphids crawl into this end of the tube, as they would have done had the tube been uncovered. If, on the other hand, the end of the tube be covered with red glass, they do not crawl into the part of the tube that is covered, unless they are very sensitive to light. Even in the latter case they may remain scattered in the red part, and do not all accumulate at the end, as they do when blue glass is used. In other words, while they respond to blue as they do to ordinary light, they behave toward red as they do towards a very faint light.

In diffuse daylight the aphids, as has been said, crawl toward the light, but if they come suddenly into the sunlight they begin to fly. Thus they remain on the food-plant until the sun strikes it, and then they fly away.

The aphid also shows another response; it is negatively geotropic, i.e. it tends to crawl upward against gravity. If placed on an inclined, or on a vertical, surface, it will crawl upward. Such an experiment is best made in the dark, since in the light the aphid also responds to the light. If put on a window it crawls upward never downward.

Aphids are also sensitive to heat. If they are placed in a darkened tube and put near a stove, they crawl away from the warmer end; but if they are acted upon by the light at the same time, they will be more strongly attracted by the light than repulsed by the heat. We thus see that there are at least three external agents that determine the movements of this animal, and its ordinary behavior is determined by a combination of these, or by that one that acts so strongly as to overpower the others.

The swarming of the male and female ants is also largely directed by the influence of light. Loeb observed that when the direct sunlight fell full upon a nest in a wall the sexual forms emerged, and then flew away. Other nests in the ground were affected earlier in the day, because the sun reached them first. These ants, when tested, were found to respond to light in the same way as do the aphids. The wingless forms, or worker ants, do not show this response, and the winged forms soon lose their strong response to light after they have left the nest. Thus we see that the heliotropism is here connected with a certain stage in the development of the individual; and this is useful to the species, as it leads the winged queens and males to leave the nest, and form new colonies. Even the loss of response that takes place later may be looked upon as beneficial to the species, since the queens do not leave the nest after they have once established it.

It is familiar to every one that many of the night-flying insects are attracted to a lamplight, and since those that fly most rapidly may be actually carried into the flame before they can turn aside, it may seem that such a response is worse than useless to them. The result must be considered, however, in connection with other conditions of their life. The following experiments carried out by Loeb on moths show some of the responses of these insects to light.

Night-flying moths were placed in a box and exposed in a room to ordinary light. As twilight approached the moths became active and began to fly always toward the window side of the box. They were positively heliotropic to light of this intensity. If let out of the case, they flew toward the window, where they remained even during the whole of the next day, fully exposed to light. If the moth is disturbed in the daytime, so that it flies, it goes always toward the light, and never away from it. These facts show that the moth is always positively heliotropic, and also that the flight toward the lamp is a natural response, misapplied in this case. That the moths do not fly by day is due to another factor, namely, the alternation in the degree of their sensitiveness at different times. But this condition alone does not seem to account fully for all the facts.

If the moths are given the alternative of flying toward the evening light, or toward the lamp, they always go toward the brighter light. Thus if, when they swarm at dusk, they are set free in the middle of the room, at the back of which a lamp is burning, the moths fly toward the window. If, however, they are set free within a metre of the lamp, they fly toward it.

The explanation that Loeb offers of the habit of these moths to fly only in the evening is, that, although they are at all times positively heliotropic, they respond to light only in the evening. In other words, it is assumed that there is a periodic change in their sensitiveness to light, which corresponds with the change from day to night. Loeb says that, just as certain flowers open only at night, and others only during the day, so do moths become more responsive in the evening, and butterflies during the day. Both moths and butterflies are positively heliotropic, and the sensitiveness of moths to light may be even greater in the evening than is that of butterflies, for the light of the evening to which the moth reacts is less than the minimal to which the butterfly responds.

Moths appear to pass into a sort of sleep during the day, while butterflies are quiescent only at night. The periodicity of the sleeping time continues, at least for several days, when the insects are kept in the dark. For instance, moths kept in the dark become restless as the evening approaches, as RÉaumur observed long ago. It has been found in plants that this sort of periodicity may continue for several days, but gradually disappears if the plants are kept in the dark. By using artificial light, and exposing the plants to it during the night, and putting them in the dark during the day, a new periodicity, alternating with the former one, may be induced; and this will continue for some days if the plants are then kept continually in the dark.

Loeb tried the experiment of exposing the quiescent moths suddenly to a lower intensity of light, in order to see if they would respond equally well at any time of day. It was found that if the change was made in the forenoon, between six o’clock and noon, it was not possible to awaken the moths by a sudden decrease in the intensity of the light. But it was possible to do so in the afternoon, long before the appearance of dusk. It appears, therefore, that in this species, Sphinx euphorbiÆ, it is possible to influence the period of awakening by decreasing the intensity of light, but this can be done only near the natural period of awakening. It seems to me that this awaking of a positively heliotropic animal by decreasing the light needs to be further investigated.

The day butterflies are also positively heliotropic. Butterflies of the species Papilio machaon, that have been raised from the pupa, remain quietly on the window in the diffuse daylight of a bright day. They can be carried around on the finger without leaving it, but the moment they come into the direct rays of the sun they fly away.

Butterflies that have just emerged from their pupa case exhibit a marked negative geotropic reaction, and this appears to be connected with the necessity of unfolding their wings at this time. Loeb says that the same cause that determines the direction of the falling stone and the paths of the planets, namely, gravity, also directs the actions of the butterfly that has just left its pupa case. The geotropic response is especially strong at first. The animal wanders around until it reaches a vertical wall, which it immediately ascends, straight upward, and remains hanging at the top until its wings have unfolded. A similar response occurs in the final stage of the larva of the May-fly, which leaves the water and crawls up a blade of grass, or other vertical support, and there, bursting the pupa skin, it dries its wings and flies away. That this is a reaction to gravity and not to light is shown by Loeb’s observation, that their empty skins are sometimes observed under a bridge where the light does not come from above. “This observation on the larva of the May-fly contradicts the assumption that the ‘purpose’ of the geotropic response of the butterfly is that it may the better unfold its new wings, for in the ephemerid larva the negative geotropism appears at a time when no wings are present.” On the other hand, it should not be overlooked that the reaction is important for the May-fly larva in other ways, because it leads the larva to leave the water at the right period, and come out into the air, where the flying insect can more safely emerge.

It is not without interest to find that caterpillars exhibit some of the same reaction shown by butterflies. Loeb has made numerous experiments with the caterpillars of Porthesia chrysorrhoea. The caterpillars of this moth collect together in the autumn and spin a web or nest in which they pass the winter. If they are taken from the nest and brought into a warm room, they will orientate themselves to the light, and also crawl toward it. If placed in a tube, they crawl to the upper side of the glass and then along this side toward the light. If a covering is placed over the end of the tube that is turned toward the window, the caterpillars will crawl only as far as the edge of the cloth. They also react negatively to gravity. If kept in a dark room, they will crawl upward to the top of the receptacle in which they are enclosed. If subjected to the influences of both light and gravity, they respond more strongly to the light. The caterpillars also show a contact reaction. They tend to collect on convex sides or on corners and angles of solid bodies. They may even pile up one on top of the other in response to this reaction; the convex side of a quiescent animal acting on another animal crawling over it as any convex surface would do and holding the animal fast.

These three kinds of reactions determine the instincts of these caterpillars. In the spring, when they become warm, they leave the nest. Positive heliotropism and negative geotropism compel them to crawl upward to the tops of the branches of the trees, and there the contact reaction with the small buds holds them fast in this place. That they are not attracted to the end of the branches by the food that they find there is shown by placing buds in the bottom of the tubes in which the caterpillars are contained. The caterpillars remain at the top of the tube, although food is within easy reach. If, however, they are placed directly on the buds, the contact reaction will hold them there, and they will not crawl farther upward. Curiously enough, as soon as the caterpillars have fed and the time for shedding approaches, the responsiveness to light and to gravity decreases, and at the time of shedding they do not respond at all to these agents. These same caterpillars react also to warmth above a certain point. In a dark tube placed near a stove, the caterpillars collect at the end farthest away from the source of the heat. They react to light best at a temperature between 20 and 30 degrees C., and above this temperature point they become restless and wander about.

The very close connection between the reactions of this caterpillar and its mode of life is perfectly obvious. The entire series of changes seems to have for its “purpose” the survival of the individual by bringing it to the place where it will find its food. It may seem natural to conclude that these responses have been acquired for this very purpose, but let us not too quickly jump at this obvious conclusion until the whole subject has been more fully examined.

The upward and downward movements of some pelagic animals have been shown to depend on certain tropic responses. Every student of marine zoology is familiar with the fact that many animals come to the surface at night, and go down at the approach of daylight. It has been shown that this migration is due largely to a response to light. Light can penetrate to only about four hundred metres in sea-water, and there is complete darkness below this level. It has been shown that the swimming larvÆ of one of the barnacles is positively heliotropic in a weak light, but negatively heliotropic in a stronger light. Animals having responses like these will come to the surface as the light fades away in the evening and remain there until the light becomes too bright in the following morning. They will then become negatively heliotropic and begin to go down. When they reach a level where the intensity of the light is such that they become positively heliotropic, they will turn and start upward again. Thus during the day they will keep below the surface, remaining in the region where they change from positive to negative, and vice versa.

It would not be difficult to imagine that this upward and downward migration of pelagic animals is useful to them, but, on the other hand, it may be equally well imagined that the response may be injurious to them. Thus it might be supposed that certain forms could procure their food by coming to the surface at night, and avoid their enemies by going down during the day. But it is difficult to see why organisms that serve as prey should not have acquired exactly the opposite tropisms in order to escape.

Some of these marine forms are also geotropic. Loeb has determined that “the same circumstances that make the animals negatively heliotropic also make them positively geotropic, and vice versa.” It was found, for instance, that the larva of the marine worm Polygordius is negatively geotropic at a low temperature, while at a higher temperature it is positively geotropic. This response would drive the animals upward when the water becomes too cold, and back again if the surface water becomes too warm; but whether the response is so adjusted that the animals keep, as far as possible, in water of that temperature that is best for their development, we do not know. We can easily imagine that within wide limits this is the case.

The change from positive to negative can also be brought about in other ways. One of the most striking cases of this sort is that described by Towle in one of the small crustaceans, Cypridopsis vidua. It was found that after an animal had been picked up in a pipette its response was always positive; that is, it swam toward the light, no matter what its previous condition had been. The disturbance caused by picking the animal up induced always a positive response towards light. If the light were moved, the Cypridopsis followed the light. In this way it could be kept positive for some time, but if it came to rest, or if it came into contact with the sides or end of the trough, it became, after a short time, negatively heliotropic, and remained negative as long as it could be kept in motion, without being disturbed, or coming into contact with a solid object. If when positive it were allowed to reach the glass at the end of the trough, it would swim about there, knocking against the glass, and then soon turn and swim away from the light. If the light were shifted while the negative animal was in the middle of the trough, it would turn and swim directly away, as before, from the source of light. It could be kept in this negative state as long as it did not come into contact with the ends.

It appears that the positive condition in Cypridopsis is of short duration, and ceases after a while either as a response to contact or without any observable external factor causing the change.

This crustacean lives at the bottom of pools, amongst water-plants, and here also, no doubt, the same change from one to the other reaction takes place. What possible advantage it may be to the animal to be kept continually changing in this way is not at all obvious, nor, in fact, are we obliged to assume that this reaction may be of any special use to it. Indeed, it is far from obvious how the change that causes the animal to swim toward the light when it is disturbed could be of the least advantage to it.

In another crustacean, one of the marine copepods, Labidocera Æstiva, it has been shown by Parker that the male and female react in a somewhat different way both to light and to gravity. The females are strongly negatively geotropic, and this sends them up to the top of the water. The males are very slightly negatively geotropic. The females are strongly positively heliotropic toward light of low intensity; the males show the same response to a less degree. To strong light the females are negative and the males are indifferent. On the other hand, the males are attracted to the females, probably in response to some chemical substance diffusing from the females, since the males show the same reaction when the females are enclosed in an opaque tube through whose ends a diffusion of substances may take place. This crustacean frequents the surface of the ocean from sunset to sunrise. During the day it retires to deeper water. Its migrations can be explained as follows: The females come to the surface at night, because they are positively heliotropic to weak light, and also because they are negatively geotropic. They go down during the day, because they react to bright light more strongly than to gravity. The males follow the females, largely because they react positively chemotactically toward the females.

Some other animals respond in a somewhat different way to light, as shown by the fresh-water planarians. These animals remain during the day under stones, where the amount of light is relatively less than outside. If they are placed in a dish in the light in front of a window, they crawl away from the light, but when they reach the back of the dish they do not come to rest, but continue to crawl around the sides of the dish even toward the light. The light makes the worms restless, and while they show a negative response as long as they are perfectly free to move away from the light, they will not come to rest when they come to the back of the dish if they are there still in the light, because the irritating action of the light on them is stronger than its directive action. If, however, in crawling about they come accidentally into a place less bright than that in which they have been, they stop, and will not leave this somewhat darker spot for a brighter one, although they might leave the newly found spot for one still less bright.

At night the planarians come out and wander around, which increases their chance of finding food, although it would not be strictly correct to say that they come out in search of food. If, however, food is placed near them, a piece of a worm, for example, they will turn toward it, being directed apparently by a sense of smell, or rather of taste.

The heliotropic responses of the planarians appear to be of use to them, causing them to hide away in the daytime, and to come out only after dark, when their motions will not discover them to possible enemies. But some of the planarians are protected in other ways, so that they will not be eaten by fish, probably owing to a bad taste; so that it is not so apparent that they are in real need of the protection that their heliotropic response brings to them. Their turning towards their food is, however, beyond question of great advantage to them, for in this way they can find food that they cannot detect in any other way.

The unicellular plants were amongst the first organisms whose tropic responses were studied, and the classical work of Strasburger gave the impetus to much of the later work. In recent years the unicellular animals, the protozoans, have been carefully studied, more especially by Jennings. His results show that the reactions in these animals are different in some important respects from those met with in higher forms. For instance, most of the free-swimming infusoria are unsymmetrical, as are also many of the flagellate forms, and as they move forward they rotate freely on a longitudinal axis. It is therefore impossible that they could orientate themselves as do the higher animals that have been described above, and we should not expect these Protozoa to react in the same way. In fact, Jennings shows that they exhibit a different mode of response. Paramoecium offers a typical case. As it moves forward it rotates toward the aboral side of the body. As a result of the asymmetry of the body, the path followed, as it revolves on its own axis, is that of a spiral. Did the animal not rotate, as it swims forward, its asymmetrical form would cause it to move in a circle, but its rotation causes, as has been said, the course to be that of a spiral, and the general direction of movement is forward.^[31] The rotation of a paramoecium on its axis is in turn caused by the oblique stroke of the cilia that cover the surface of the body. Their action when reversed causes the animal to rotate backward.

If a drop of weak acid be put into the water in which the paramoecia are swimming,—for instance, in the water between a cover-slip and a slide,—it will be found, after a time, that many individuals have collected in the drop. It was at first supposed that the paramoecia are attracted by the diffusion of the acid in the water, and turn toward the source of the chemical stimulus; but Jennings has shown that this is not the way in which the aggregation is brought about. If the individuals are watched, it will be found that they swim forward in a spiral path without regard to the position of the drop of acid. If one happens, by chance, to run into the drop, there is no reaction as it enters, but when it reaches the other side of the drop, and comes into contact with the water on this side, it suddenly reacts. It stops, backs into the middle of the drop, rotates somewhat toward the aboral side (i.e. away from the vestibule), and then starts forward again, only to repeat the action on coming into contact with the edge of the drop again. The paramoecium has been caught in a veritable trap. All paramoecia that chance to swim into the drop will also be caught, until finally a large number will accumulate in the region. The result shows, that, in passing from ordinary water into a weak acid, no reaction takes place; but having once entered the acid, the animal reacts on coming into contact with the water again.

On the other hand, there are some substances to which the paramoecium may be said to be negatively chemotropic. If a drop of a weak alkaline solution be put into water in which paramoecium is swimming, an individual that happens to run against it reacts at once. It stops instantly, backs off, revolving in the opposite direction, turns somewhat to one side, and swims forward again. The chances are that it will again hit the drop, in which case it repeats the same reaction, turning again to one side. If it continues to react in this way, it will, in the course of time, turn so far that when it swims forward it will miss the edge of the drop, and then continue on its way. If an individual were put into an alkaline drop, it would leave it, because it would not react when it passed from inside the drop into the surrounding water.

Unicellular animals react to other things besides differences in the chemical composition of different parts of a solution. In many cases they react to light, swimming toward or away from it according to whether they are positively or negatively heliotropic. If they are positively heliotropic, and while swimming run into a shadow, they react as they would on coming into contact with a drop of acid. Since they rotate as they swim forward, we cannot explain their orientation as in the case of other animals that hold a fixed vertical position. If we assume that the two ends of the body are differently affected by the light, for which there is some evidence, we can perhaps in this way account for their turning toward, or away from, the source of light.

Changes in the osmotic pressure of the different parts of the fluid, mechanical stimulation produced by jarring, extremes of heat and of cold, all cause this same characteristic reaction in Paramoecium; and this accounts for their behavior toward these agents that are so different in other respects.

Paramoecia, as well as other protozoans, show a contact response. They fix themselves to certain kinds of solid bodies. If, for example, a small bit of bacterial slime is put into the water, the paramoecia collect around it in crowds, and eat the bacteria; but they will collect in the same way around almost any solid. On coming in contact with bodies having a certain physical texture, the cilia covering the paramoecium stop moving, only those in the oral groove continuing to strike backward. The animal comes to rest, pressed against the solid body. If one or more paramoecia remain in the same place, they set free carbon dioxide, as a result of their respiratory processes. There is formed around them a region containing more of this acid than does the surrounding water. If other moving paramoecia swim, by chance, into this region, they are caught, and as a result an accumulation of individuals will take place. The more that collect the larger will the area become, and thus large numbers may be ultimately entrapped in a region where there is formed a substance that, from analogy with other animals, we should expect to be injurious.

The question as to how far these responses of the unicellular forms are of advantage to them is difficult to decide, for while, as in the above case, the response appears to be injurious rather than useful, yet under other conditions the same response may be eminently advantageous. In other cases, as when the paramoecia back away, and then swim forward again, only to repeat the process, the act appears to be such a stupid way of avoiding an obstacle that the reaction hardly appears to us in the light of a very perfect adaptation. If we saw a higher animal trying to get around a wall by butting its head into it until the end was finally reached, we should probably not look upon that animal as well adapted for avoiding obstacles.

Bacteria, which are generally looked upon as unicellular plants, appear, despite the earlier statements to the contrary, to react in much the same way as do the protozoans, according to the recent work of Rothert, and of Jennings and Crosby. The bacteria do not seem to turn toward or away from chemical substances, but they collect in regions containing certain substances in much the same way as do the protozoans. The collecting of bacteria in regions where oxygen is present has been known for some time, but it appears from more recent results that they are not attracted toward the oxygen, but by accidentally swimming into a region containing more oxygen they are held there in the same way as is paramoecium in a drop of acid. On the other hand bacteria do not enter a drop of salt solution, or of acids, or of alkalies. They react negatively to all such substances. Some kinds of bacteria have a flagellum at each end, and swim indifferently in either direction. If they meet with something that stimulates them, as they move forward, they swim away in the opposite direction, and continue to move in the new direction until something causes again a reversal of their movement. In this respect their mode of reaction seems of greater advantage than that followed by paramoecium.

Another instinct, that appears to be due to a tropic response, is the definite time of day at which some marine animals deposit their eggs. The primitive fish, Amphioxus, sets free its eggs and sperm only in the late afternoon. A jellyfish, Gonionema, also lays its eggs as the light begins to grow less in the late afternoon, and in this case it has been found that the process can be hastened if the animals are placed in the dark some hours before their regular time of laying. There is no evidence that this habit is of any advantage to the animal. We may imagine, if we like, that the early stages may meet with less risk at night, but this is not probable, for it is at this time that countless marine organisms come to the surface, and it would seem that the chance of the eggs being destroyed would then be much greater. It is more probable that the response is of no immediate advantage to the animals that exhibit it, although in particular cases it may happen to be so.

This response recalls the diurnal opening and closing of certain flowers. The flowers of the night-blooming cereus open only in the dusk of evening, and then emit their strong fragrance. Other flowers open only in the daytime, and some only in bright sunlight. It is sometimes pointed out that it is of advantage to some of these flowers to open at a certain time, since the particular insects that are best suited to fertilize them may then be abroad. This may often be the case, but we cannot but suspect that in other cases it may be a matter of little importance. In special instances it may be that the time of opening of the flowers is of importance to the species; but even if this is so, there is no need to assume that the response has been gradually acquired for this particular purpose. If it were characteristic of a new form to open at a particular time, and there were insects in search of food at this time that would be likely to fertilize the plant, then the plant would be capable of existing; but this is quite different from supposing that the plant developed this particular response, because this was the most advantageous time of day for the fertilization of its flowers.

We can apply this same point of view, I believe, to many of the remarkable series of tropisms shown by plants, whose whole existence in some cases is closely connected with definite reactions to their environment. Let us examine some of these cases.

When a seed germinates, the young stem is negatively geotropic, and, in consequence, as it elongates it turns upward towards the light that is necessary for its later growth. The root, on the contrary, is positively geotropic, and, in consequence, it is carried downward in the ground. Both responses are in this case of the highest importance to the seedling, for in this way its principal organs are carried into that environment to which they are especially adapted. It matters very little how the seed lies in the ground, since the stem when it emerges will grow upward and the root downward. The young stem, when it emerges from the soil, will turn toward the light if the illumination comes from one side, and this also may often be of advantage to the plant, since it turns toward the source from which it gets its energy. The leaves also turn their broad surfaces toward the light, and as a result they are able to make use of a greater amount of the energy of the sunlight. The turning is due to one side of the stem growing more slowly than the opposite side, and it is true, in general, that plants grow faster at night than in the daylight. Very bright light will in some cases actually stop all growth for a time. Thus we see that this bending of the stem toward the light and the turning of the leaves to face the light are only parts of the general relation of the whole plant toward the light.

Negative heliotropism is much less frequent in plants. It has been observed in aËrial roots, in many roots that are ordinarily buried in the ground, in anchoring tendrils that serve as holdfasts, and even in the stems of certain climbers. In all of these cases, and more especially in the case of the climbers, the reaction is obviously of advantage to the plant; and it is significant to find, in plants that climb by tendrils carrying adhering disks, that there is a reversal of the ordinary heliotropism shown by homologous organs in other plants. There is an obvious adaptation in the behavior of the tendril, since its growth away from the more illuminated side is just the sort of reaction that is likely to bring it into contact with a solid body.

In this connection it is important to observe that these reactions to light are perfectly definite, being either positive or negative under given conditions, and therefore there is at present nothing to indicate that there has been a gradual transformation from positive to negative, or vice versa. It seems to me much more probable that when the structural change took place, that converted the plant into a climber, there appeared a new heliotropic response associated with the other change. In other words, both appeared together in the new organ, and neither was gradually acquired by picking out fluctuating variations.

The leaves of plants also show a sort of transverse heliotropic response. It has been found, for example, that the leaves of Malva will turn completely over if illuminated by a mirror from below. A curious case of change of heliotropism is found in the flower stalks of Linaria. They are at first positively heliotropic, but after the flower has been fertilized the stalk becomes negatively heliotropic. As the stalks continue to grow longer, they push the fruits into the crevices of the rocks on which the plants grow, and in this way insure the lodgement of the seeds. Here we have an excellent example showing that the negative heliotropism of the flower stalk could scarcely have been acquired by slight changes in the final direction, for only the complete change is useful to the plant. Intermediate steps would have no special value.

As has been pointed out in the case of the seedling plant, the main stem responds positively and the roots negatively to gravity. In addition to this, the lateral position taken by the lateral roots and branches and by underground stems are also, in part, due to a geotropic response. In this case also the effect is produced by the increased growth on the upper side when the response is positive, and on the lower when it is negative. Leaves also assume a transverse position in response to the action of gravity, or at least they make a definite angle with the direction of its action.

The most striking case of geotropic response is seen in plants that climb up the stems of other plants. The twining around the support is the result of a geotropic response of the sides of the stem. The young seedling plant stands at first erect. As its end grows it begins to curve to one side in an oblique position, and this is due to an increase in growth on one side of the apex of the shoot. As a result the stem bends toward the other side. Not only does the end “sweep round in a circle like the hands of a watch,” but it rotates on its long axis as it revolves. As a result of this rotation “the part of the stem subjected to the action of the lateral geotropism is constantly changing; and the revolving movement once begun, must continue, as no position of equilibrium can be attained.” This movement will carry the end around any support, not too thick, that the stem touches.

Most climbers turn to the left, i.e. against the hands of a watch, others are dextral, and a few climb either way.^[32] Strasburger states that whenever any external force, or substance, is important to the vital activity of the plant or any of its organs, there will also be found to be developed a corresponding irritability to their influence. Roots in dry soil are diverted to more favorable positions by the presence of greater quantities of moisture. This may, I venture to suggest, be putting the cart before the horse. The plant may be only able to exist whose responses are suited to certain external conditions, and these determine the limits of distribution of the plant or the places in which it is found.

A number of plants climb in a different way, and show another sort of tropism. Those that climb by means of tendrils twist their tendrils about any support that they happen to come in contact with, and thus the plant is able to lift its weak stem, step by step, into the air. The twining of the tendrils is due to contact, which causes a cessation of growth at the points of contact. The growth of the opposite side continues, and thus the tendril bends about its support. In the grape and in ampelopsis the tendril is a modified branch. The stalk of the leaves in a few plants, as in Lophospermum, act as tendrils. Other climbers are able to ascend vertical walls owing to the presence of disks, whose secretions hold the tendril firmly against the support, as in ampelopsis.

It is interesting to find in practically all these cases that, whatever the stimulus may be, the results are reached in the same way, namely, by one part growing faster than another. The fact of importance in this connection is that the plant is so constructed that the response is often beneficial to the organism.

Before leaving this subject there is one set of responses to be referred to that is not the result of growth. Certain movements are brought about by the change in the turgidity of certain organs. The small lateral leaflets of Desmodium gyrans make circling movements in one to three minutes. No apparent benefit results from their action. The terminal leaflets of Trifolium pratense oscillate in periods of two to four hours, but do so only in the dark; in the light the leaflets assume a rigid position. There is nothing in the process to suggest that the movement is useful to the plant, and yet it appears to be as definite as are those cases in which the response is of vital importance. Had these movements been of use, their origin would, no doubt, have been explained because of their usefulness, and the conclusion would have been wrong.

The leaves of the Mimosa respond, when touched, and it cannot be supposed that this is of any great advantage to the plant. The sleep movements of many plants are also due to the effect of light. In some cases the leaflets are brought together with their upper surfaces in contact with one another; in other cases the lower surfaces are brought together. Darwin supposed that these sleep movements served to protect the leaves from a too rapid loss of heat through radiation, but it has been pointed out that tropical plants exhibit the same responses. We have here another admirable instance of the danger of concluding that because we can imagine an advantage of a certain change, that the change has, therefore, been acquired because of the advantage. In the Mimosa not only do the leaflets close together, but the whole leaf drops down if the stimulus is strong. Other plants also show in a less degree the same movements, Robinia and Oxalis for instance, and certainly in these latter the result does not appear to be of any advantage to the plants.

The preceding account of some of the tropisms in animals and plants will serve to give an idea of how certain movements are direct responses to the environment. Some of the reactions appear to be necessary for the life of the individual, others seem to be of less importance, and a few of no use at all. Yet the latter appear to be as definite and well-marked as are the useful responses. I think the conviction will impress itself on any one who examines critically the facts, that we are not warranted in applying one explanation to those responses that are of use, and another to those that are of little or of no value. Inasmuch as the Darwinian theory fails to account for the origin of organs of little or of no value, it is doubtful if it is needed to explain the origin of the useful responses. If, on the other hand, we assume that the origin of the responses has nothing to do with their value to the organism, we meet with no difficulty in those cases in which the response is of little or of no use to the organism. That great numbers of responses are of benefit to the organism that exhibits them can be accounted for on the grounds that those new species, that have appeared, that have useful responses, are more likely, in the long run, to survive, than are those that do not respond adaptively.

We may now examine some of the more complicated responses and instincts, more especially those of the higher animals. Some of these are pure tropisms, i.e. definite responses or reactions to an external exciting agent; others may be, in part, the result of individual experience, involving memory; others, combinations of the two; and still others may depend on a more complex reaction in the central nervous system of the animal. These cases can be best understood by means of a few illustrations.

As an example of a simple action may be cited a well-known reflex after cutting the nerve-cord of the frog, or after destroying the brain. If the frog is held up, and its side tickled, the leg is drawn up to rub the place touched. To accomplish this requires a beautifully adjusted system of movements, yet the act seems to be a direct reflex, involving only the spinal cord.

An example of a somewhat more complex reflex is the biting off of the navel-string by the mother in rodents and other mammals; an act eminently useful to the young animal, although of no importance to the mother herself. The protection of the young by their parents from the attacks of other animals appears to be a somewhat complex instinct, and it is interesting to note that the protection is extended to the young only so long as they are in need of it, and as soon as they are able to shift for themselves the maternal protection is withdrawn.

The instinct of the young chick to seize in its beak any small moving object is a simple and useful reflex action, but if the object should happen to be a bee which stings the chick, another bee or similar insect will not be seized. Here we see that a reflex has been changed, and changed with amazing quickness. Moreover, the chick has learnt to associate this experience with a particular sort of moving object. It is this power to benefit by the result of a brief experience that is one of the most advantageous properties of the organism.

Young chicks first show a drinking reflex if by chance their beaks are wet by water. At once the head is lifted up, and the drop of water passes down the throat. In this way the chick first learns the meaning of water, and no doubt soon comes to associate it with its own condition of thirst. The sight of water produces no effect on the inexperienced chick, and it may even stand with its feet in the water without drinking; but as soon as it touches, by chance, the water with its beak, the reflex, or rather the set of reflexes is started.

A more complicated instinct is that shown by the spider in making its web. In some cases the young are born from eggs laid in the preceding summer, and can have had, therefore, no experience of what a web is like; and yet, when they come to build this wonderfully complex structure, they do so in a manner that is strictly characteristic of the species.

The formation of the comb by bees, in which process, with a minimum of wax, they secure a maximum number of small storehouses in which to keep their honey and rear their young, is often cited as a remarkable case of adaptation.

There has been some discussion as to whether birds build their nests in imitation of the nest in which they were reared, or whether they do so independently of any such experience. There can be no doubt, however, that in some birds neither memory nor imitation can play any important part in the result, and that they build their nests as instinctively as spiders make webs.

These instincts of spiders, bees, and birds appear to be more complex than the reflexes and tropisms that were first described. Whether they are really so, or only combinations of simple responses, we do not yet know. That they have come suddenly into existence as we now find them does not seem probable, but this does not mean that they must have been slowly acquired as the result of selection. The mutation theory also assumes that the steps of advance may have been small.

Our account may be concluded with the recital of some instincts, chosen almost at random, that serve to show some other adaptations which are the result of these inborn responses.

It is known that ants travel long distances from their nests, and yet return with unerring accuracy. It has been shown that they are able to do this through a marvellous sense of smell. The track left by the ant, as it leaves the nest, serves as a trail in returning to the starting-point. Moreover, it appears that the ant can pick out her own trail, even when it has been crossed by that of other ants. This means that she can distinguish the odor of her own trail from that of other members of the colony. The sense-organs by means of which the odor is detected lie in the antennÆ. This fact accounts for certain actions of ants that have been described as showing that they have an affection for each other. Two ants, meeting, pat each other with their antennÆ. In this way they are quickly able to distinguish members of their own nest from those of other nests. If they are of the same nest, they separate quietly; if of other nests, they may fight. If an ant from one nest is put into another nest, it is instantly attacked and killed—an act that appears to be injurious rather than useful, for the ant might become a valuable member of the new colony. If, however, an ant is first immersed in the blood of a member of the community into which she is to be introduced, she will not be attacked, and may soon become a part of the new community. By her baptism of blood she has no doubt acquired temporarily the odor of the new nest, and by the time that this has worn off she will have acquired this odor by association, and become thereby a member of another colony.

Numerous stories have been related of cases in which an ant, having found food, returns to the nest with as much of it as she can carry, and when she comes out again brings with her a number of other ants. This has been interpreted to mean that in some mysterious way the ant communicates her discovery to her fellow-ants. A simpler explanation is probably more correct. The odor of the food, or of the trail, serves as a stimulus to other ants, that follow to the place where the first ant goes for a new supply of the food. The fact that the first individual returns to the supply of food seems to indicate that the ant has memory, and this is obviously of advantage to her and to the whole colony.

The peculiar habits of some of the solitary wasps, of stinging the caterpillar or other insect which they store up as food for their young, is often quoted as a wonderful case of adaptive instinct. The poison that is injected into the wound paralyzes the caterpillar, but as a rule does not kill it, so that it remains motionless, but in a fresh state to serve as food for the young that hatch from the egg of the wasp. A careful study of this instinct by Mr. and Mrs. Peckham has shown convincingly that the act is not carried out with the precision formerly supposed. It had been claimed that the sting is thrust into the caterpillar on the lower side, a ventral ganglion being pierced, the poison acting with almost instantaneous effect. But it may be questioned whether this is really necessary, and whether the same end might not be gained, although not quite so instantaneously, if the caterpillar were pierced in almost any other part of the body. Can we be seriously asked to believe that this instinct has been perfected by the destruction of those individuals (or of their descendants) that have not pierced the caterpillar in exactly the middle of a segment of the anterior ventral surface? It seems to me that the argument proves too much from the selectionist’s point of view. If the wasp pierced the caterpillar in the middle of its back, we should have passed over the act without comment; but since the injection is usually made on the ventral side, and since we know that the nervous system lies in this position, it has been assumed that the act is carried out in this way, in order that the poison may penetrate the nervous system more quickly. Yet a fuller knowledge may show that there is really no necessity for such precision.

A curious response is the so-called death-feigning instinct shown by a number of animals, especially by certain insects, but even by some mammals and birds. Certain insects, if touched, draw in their legs, let go their hold, and fall to the ground, if they happen to be on a plant. It is not unusual to meet with the statement that this habit has been acquired because it is useful to the insect, since it may often escape in this way from an enemy. This does not appear on closer examination to be always the case, and sometimes as much harm as good may result, or what is more probable, neither much advantage, nor disadvantage, is the outcome. This can, of course, only be determined in each particular case from a knowledge of the whole life of a species and of the enemies that are likely to injure it.

Hudson has recorded^[33] a number of cases of this death-feigning instinct in higher animals, and attributes it to violent emotion, or fear, that produces a sort of swoon. He describes the gaucho boys’ method, in La Plata, of catching the silver-bill by throwing a stick or a stone at it, and then rushing toward the bird, “when it sits perfectly still, disabled by fear, and allows itself to be taken.” He also states that one of the foxes (Canis azarÆ) and one of the opossums (Didelphys azarÆ) “are strangely subject to the death-simulating swoon.”

33.“The Naturalist in La Plata.”