Several kinds of tropisms are recognized, such as phototropism or heliotropism, reaction to light; thermotropism, reaction to heat; electrotropism or galvanotropism, to electric current; geotropism, to gravity; chemotropism, to a chemical; rheotropism, to current; thigmotropism or stereotropism, to contact; and chromotropism, to color.
Many Animals Show Tropic Responses.—Many of the lower animals seem to have their movements determined more or less mechanically by the action of such external factors, some being positively, others negatively responsive to a given kind of stimulus, or the same individual may be at one time positive, at another negative, according to modifying conditions to be mentioned presently.
In plants and in simpler lower animals there is no special nervous system. The responses of these organisms depend on the general irritability of their constituent protoplasm. In other animals a nervous system is developed, crude and diffuse in lower forms, extremely delicate, complex and definitely ordered in higher forms. But it should be borne in mind that nerve protoplasm possesses only in high degree a capacity for irritability, conduction, etc., that is common to all living substance. In keeping with other “physiological divisions of labor” or specialization which mark the increasing complexity of animals, this enormously enhanced sensitivity and conductivity of certain tissues have come about, and they have become set apart for these special functions. In higher animals, therefore, the tropisms where operative must act more or less through the agency of the nervous system instead of directly through the general protoplasm of the organism.
Certain Apparently Complex Volitions Probably Only Tropisms.—Where nervous systems enter into tropic responses there must be specific sensibility of certain nerve terminations (i. e., sense organs) at the surface of the body. These sensory or receiving nerves connect through the central system with corresponding motor nerves which in turn supply certain specific muscles through the contraction of which the organism is as surely and as mechanically oriented as in the simpler cases. For example, if light is the stimulating agent, when it strikes a positively phototropic animal, if the latter is not already oriented, the eyes or other nerve terminations sensitive to light transmit an impulse through the central nervous system to certain muscles causing them to increase their tension and thereby swing the animal around with its head toward the light. Progressive movements which the organism then makes must carry it toward the source of light. Thus it is not “love of light” that draws the moth into the flame but the mechanical steering of the body toward the source of light through the stimulations produced by the light waves. It is chemotropism, not solicitude for its offspring, which drives the flesh fly to lay its eggs on decaying meat. And it is stereotropism and not a desire for concealment which impels certain animals such as many worms and insects to get into a close contact with solid bodies, or in other words to “hide” themselves in burrows and crevices.
Complicating Factors.—However, beautifully as these theories of tropisms work out in a broad general way, there are various additional factors entering which must be reckoned with, and these become more numerous and of more consequence as the organism becomes more complex. In the first place certain internal conditions must be considered. Living matter is characterized by its instability. There are continual synthetic and disruptive processes in progress which the physiologist terms metabolic changes. The very “life” of such matter seems to be the manifestation of such changes. Concerning what the ultimate source of these changes is, whether or not indirectly they may be referred to external conditions as seems probable to many biologists, no one so far has ever given a convincing, positive answer. It is sufficient for our purposes to know that they may have set up certain internal stimuli which may modify the behavior of the organism in which they reside, and that the “physiological state” of the organism at the time of external or internal stimulation will condition the response. This physiological condition may be dependent on the general metabolic equilibrium of the animal, or on the extent of previous stimulation by means of the same or different agents. Thus the organism may not always react in the same way to the same stimulus.
The intensity of the stimulation and change in the intensity of the stimulation, are also factors to be reckoned with. Moreover, it must be taken into account that a given organism is often operating under the control of more than one external influence. For example, swarm spores in a dish of water which at a given temperature are positively phototropic, that is, gather at the side of the dish toward the light, may, if the temperature of the water is raised or in case of marine forms if the salinity is increased, become negatively phototropic. Sometimes two or more forms of stimuli may cooperate in bringing about certain behavior as, for instance, in the reaction of the earthworm to a suitable habitat, through a combination of chemical and contact stimuli. On the other hand, two different stimuli may interfere with each other; for example, the usual phototropic responses of certain animals do not manifest themselves when they are mating or feeding. In short, anything that alters the physiological state of the organism may cause it to react in a different manner. And thus with the interplay of shifting external agents and variable internal state the bounds of behavior on these purely mechanical bases become considerably extended.
Many Tropic Responses Apparently Purposeful.—The query arises as to why if these responses are mechanical they are so often apparently purposive; that is, why do they so often subserve some useful end for the animal? While they do not always work out to the animal’s benefit, as for instance in the case of the moth and the light or under many other conditions that can be devised experimentally, as a matter of fact under normal natural conditions they are on the whole useful to the organism, carrying it into suitable surroundings of food, lessened danger, temperature, and the like.
The probabilities are that in their first origin the reactions were not purposive. However, if any proved harmful they would result in the extermination of their possessors and hence of that particular strain of individuals. Those types that happened to have useful reactions would be left and in course of time as the process of eliminating the others went on, would become the prevailing types. Any organism which the useful reaction had preserved would tend to hand it down to the succeeding generation where again it would be the conserver of those individuals which possessed it in sufficient degree.
Authorities Not Agreed on Details of Tropic Responses.—Although all the foremost modern students of animal behavior accept as facts the more or less mechanical orienting effects of external stimuli, there is by no means unanimity of opinion regarding details. Some stress as the directive factor the continuous action of the stimulating agent on sensitive tissues symmetrically situated. Others would maintain that it is the time rate of change in the intensity of the stimulating agent, or that the factor is different in different cases. Some make much of an automatic sort of “trial and error” system by which certain organisms test out an inimical environment until the path of least irritation is hit upon as the way to safety. The field is a broad one and to get at the finer shades of distinction the reader will have to refer to the works of such authorities as Loeb, Jennings, Holmes and Mast.
Tropisms Grade Into Reflex Actions and Instincts.—The tropisms in many cases become indistinguishable from reflex actions and these in turn grade up into the instincts of animals. The latter may be looked on as but subtler and more involved reactions made possible through a more intricate structural organization. As might be expected of instincts, the feature of utility is more in evidence than in simpler tropisms because they have become of proportionately greater magnitude, but the same fundamental mechanism is apparently at bottom of both. It has already been seen how the “instinct” of the blow-fly to lay its egg on meat is interpretable as a chemotropic response. Thus no elaborate psychic mechanism is necessary in such behavior.