The instinct of the most sagacious ant. bee, or bird may be, and is, occasionally at fault.; birds may in certain eases change their nesting habits. On the other hand, certain spiders, insects, as well as birds and mammals, are known to 'rise with the occasion' to overcome obstacles, and to he guided, as we say; by the germs of reason to act much as a human being would under similar circumstances.
There are three grades of mentality; i.e. three steps in the evolution of mind. These are: (1) Reflex acts and tropisms; (2) instinctive acts; and (3) acts of reason. It is, as everybody knows, difficult to draw the line of division be tween these grades. It may be said, however, that what we call instincts are on the whole in termediate between the simplest physiological or primary reflex acts, especially those resulting from some external stimulus of light, heat, grav ity, odors, and sounds; i.e. the various tropisms (sec Tnomsm), and nets of intelligence and of reason. In short, animals are neither automata, nor are they as a rule guided by intelligence or reason, as in man.
Let us first consider what are mere physio logical or reflex acts; i.e. tropisms. One example is the efforts of the young chick at pecking food. This is said by Lloyd to be "a motor re sponse to a certain stimulus," which is purely mechanical or organic, and is unaccompanied by consciousness. Loeb defines a reflex as "a re action which is caused by an external stimulus." It is well known that plants, and also the lowest or simplest animals, turn to the light, are helio tropic; that they also respond to the stimulus of cold, heat, hunger, and reproductive needs. They are also geotropic. They move in certain
directions, take in food as if exercising the power of choice, though this is not the case. Thus the reflex acts which lie at the base of in stinetive acts are, says Loeb, the various trop isms. In this respect the lowest animals (proto zoans, sponges, and polyps) are scarcely if any higher than plants. In the higher animals, as in man, winking is a reflex act, the closing of the eyelids is a reaction caused by an external stimu lus, as when the conjunctiva is touched by a for eign body, or when the pupil is narrowed under the influence of light ( Loeb).
The result of experiments on the reactions of plants and animals to various external stimuli shows that irritability and conductibility "are the only qualities essential to reflexes," and it will be remembered that these are the common qualities of the protoplasm both of plants and animals. "The irritable structures at the sur face of the body and the arrangement of the muscles determine the character of the reflex acts" (Loeb).
Reflex acts apparently have a purposive char acter. Examples of reflex acts in the whole ani mal, the individual (not simply in distinct or gans), are those actions related to the outer world and other organisms, manifested by proto zoans, such as monads and ameba:, by sponges and polyps, as well as the ascidians. Their move ments, modes of escaping danger, of getting food, or their deportment in freely moving forms in the presence of their prey or their enemies, their mode of conjugation, are acts in response either to external stimuli, such as light, heat or cold, odors, tastes, smells, or the effects of hunger or the reproductive functions.