only to be expected that this useful modification of colors and patterns, that results in the striking phenomena of special protective resemblances and mimicry, should be specially widespread and pronounced among insects. Moreover, they are mostly deficient in other means of defense, and seem to be the favorite food for many different kinds of animals. Protective resemblance is their best and most widely adopted means of preserving life. 114. No volition in mimicry.-The use of the word mimicry has been criticised because it suggests the exercise of volition or intent on the part of the mimicking animal. The student should not entertain this conception of mimicry. In the use of "mimicry" in connection with the phenomena just described, the biologist ascribes to it a technical meaning, which excludes any suggestion of volition or intent on the part of the mimic. Just how such extraordinary and perfect cases of mimicry as shown by Phyllium and Kallima have come to exist is a problem whose solution is not agreed on by naturalists, but none of them makes volition-the will or intent of the animal-any part of his proposed solution. Each case of mimicry is the result of a slow and gradual change, through a long series of ancestors. The mimicry may indeed include the adoption of certain habits of action which strengthen and make more pronounced the deception of shape and color. But these habits, too, are the result of a long development, and are instinctive or reflex-that is, performed without the exercise of volition or reason. 115. Color; its utility and beauty. The causes of color, and the uses of color in animals and in plants are subjects to which naturalists have paid and are paying much attention. The subject of "protective resemblances and mimicry" is only one, though one of the most interesting, branches or subordinate subjects of the general theory of the uses of color. Other uses are obvious. Bright colors and markings may serve for the attraction of mates; thus BRYANT WALKER LIBRARY MUSEUM OF ZOOLOGY PROTECTIVE RESEMBLANCES, AND MIMICRY 223 are explained by some naturalists the brilliant plumage of the male birds, as in the case of the bird-of-paradise and the pheasants. Or they may serve for recognition characters, enabling the individuals of a band of animals readily to recognize their companions; the conspicuous whiteness of the short tail of the antelopes and cotton-tail rabbits, the black tail of the black-tail deer, and the white tailfeathers of the meadow-lark, are explained by many naturalists on this ground. Recognition marks of this type are especially numerous among the birds, hardly a species being without one or more of them, if their meaning is correctly interpreted. The white color of arctic animals may be useful not alone in rendering them inconspicuous, but may serve also a direct physiological function in preventing the loss of heat from the body by radiation. And the dark colors of animals may be of value to them in absorbing heat rays and thus helping them to keep warm. But "by far the most widespread use of color is to assist an animal in escaping from its enemies or in capturing its prey." The colors of an animal may indeed not be useful to it at all. Many color patterns exist on present-day birds simply because, preserved by heredity, they are handed down by their ancestors, to whom, under different conditions of life, they may have been of direct use. For the most part, however, we can look on the varied colors and the striking patterns exhibited by animals as being in some way or another of real use and value. We can enjoy the exquisite coloration of the wings of a butterfly none the less, however, because we know that these beautiful colors and their arrangement tend to preserve the life of the dainty creature, and have been produced by the operation of fixed laws of Nature working through the ages. CHAPTER XIII THE SPECIAL SENSES 116. Importance of the special senses.-The means by which animals become acquainted with the outer world are the special senses, such as feeling, tasting, smelling, hearing, and seeing. The behavior of animals with regard to their surroundings, with regard to all the world outside of their own body, depends upon what they learn of this outer world through the exercise of these special senses. Habits are formed on the basis of experience or knowledge of the outer world gained by the special senses, and the development of the power to reason or to have sense depends on their pre-existence. 117. Difficulty of the study of the special senses. We are accustomed to think of the organs of the special senses as extremely complex parts of the body, and this is certainly true in the case of the higher animals. In our own body the ears and eyes are organs of most specialized and highly developed condition. But we must not overlook the fact that the animal kingdom is composed of creatures of widely varying degrees of organization, and that in any consideration of matters common to all animals those animals of simplest and most lowly organization must be studied as well as those of high development. The study of the special senses presents two phases, namely, the study of the structure of the organs of special sense, and the study of the physiology of special sense-that is, the functions of these organs. It will be recognized that in the study of how other animals feel and taste and smell and hear and see, we shall have to base all our study on our own experience. We know of hearing and seeing only by what we know of our own hearing and seeing; but by examination of the structure of the hearing and seeing organs of certain other animals, and by observation and experiments, zoologists are convinced that some animals hear sounds that we can not hear, and some see colors that we can not see. 66 While that phase of the study of the special senses which concerns their structure may be quite successfully undertaken, the physiological phase of the study of the actual tasting and seeing and hearing of the lower animals is a matter of much difficulty. The condition and character of the special senses vary notably among different animals. There may even exist other special senses than the ones we possess. Some zoologists believe that certain marine animals possess a density or pressure sense that is, a sense which enables them to tell approximately how deep in the water they may be at any time. To certain animals is ascribed a "temperature sense," and some zoologists believe that what we call the homing instinct of animals as shown by the homing pigeons and honey-bees and other animals, depends on their possession of a special sense which man does not possess. Recent experiments, however, seem to show that the homing of pigeons depends on their keen sight. In numerous animals there exist, besides the organs of the five special senses which we possess, organs whose structure compels us to believe them to be organs of special sense, but whose function is wholly unknown to us. Thus in the study of the special senses we are made to see plainly that we can not rely simply on our knowledge of our own body structure for an understanding of the structure and functions of other animals. 118. Special senses of the simplest animals.--In the Amaba (see Chapter I), that type of the simplest animals, with one-celled body, without organs, and yet with its capacity for performing the necessary life processes, there are no special senses except one (perhaps two). The Amaba can feel. It possesses the tactile sense. And there are no special sense organs except one, which is the whole of the outer surface of the body. If the Amoeba be touched with a fine point it feels the touch, for the soft viscous protoplasm of its body flows slowly away from the foreign object. The sense of feeling or touch, the tactile sense, is the simplest or most primitive of the special senses, and the simplest, most primitive organ of special sense is the outer surface or skin of the body. Among those simple animals that possess the simplest organs of hearing and perceiving light, we shall find these organs to be simply specialized parts of the skin or outer cell layer of the body, and it is a fact that all the special sense organs of all animals are derived or developed from the outer cell layer, ectoblast, of the embryo. This is true also of the whole nervous system, the brain and spinal cord of the vertebrates, and the ganglia and nerve commissures of the invertebrates. And while in the higher animals the nervous system lies underneath the surface of the body, in many of the lower, many-celled animals all the ganglia and nerves, all of the nervous system, lie on the outer surface of the body, being simply a specialized part of the skin. 119. The sense of touch.-In some of the lower, manycelled animals, as among the polyps, there are on the skin certain sense cells, either isolated or in small groups, which seem to be stimulated not alone by the touching of foreign substances, but also by warmth and light. They are not limited to a single special sense. They are the primitive or generalized organs of special sense, and can develop into specialized organs for any one of the special senses. The simplest and most widespread of these special senses with, as a whole, the simplest organs, is the tactile |