Critical consideration of color stimuli and of the nature of animal response has indicated, during the past few years, that colored objects may be responded to specifically for various reasons, important among which are (1) color; (2) lightness or intensity of visual stimulation; (3) pattern or texture of the stimulating sur face; (4) apparent distance from the eye; (5) size; (6) form or shape; (7) odor or tape; (8) temperature or electrical condition; (9) tactual.quality, and so on.
Only if the possibility of response to all other sensory factors is eliminated or controlled can the observer be certain of color vision.
To-day, instead of using two colored balls or bits of colored paper, the skilled and cau tious animal, psychologist demands for his study of color vision adequate physical means of ob taining pure color stimuli, of controlling them in their various aspects and of measuring both stimuli and response.
Modern experiments indicate that color vision is less widely distributed among animals than has been supposed. Mice and rats, if not also other rodents, possess little or no ability to distinguish colors. Cats and dogs, if not color blind, possess color senses radically differ ent from the human. The color vision of bees, wasps and ants is very imperfectly known. In certain birds, color sensitivity is highly de veloped. This is true, likewise, of monkeys, anthropoid apes and probably of several other mammals. • What has happened in the case of color vision is happening also for other senses. The thorough study of the senses of any particular animal requires infinite time and patience. Sight, hearing, smell, taste, touch, internal senses must be studied before the life of an animal can be adequately described,— indeed, before certain of the most important and inter esting problems of habit formation can be solved. The psychology of the senses is the foundation of the modern science of animal psychology. It is difficult to exaggerate its importance.
The Role of the Senses: Orientation.— When it is asked, how an animal can recognize and satisfactorily respond to a particular object or situation, the senses immediately come to mind, for it is their chief role to supply the guiding factors of response. So apparently simple a reaction as orientation with respect to a source of light is controlled, doubtless, by sensory processes. In fact, all forms of orienta
tion to near and distant objects would appear to be conditioned by the various senses. Thus, the carrier pigeon flies many miles to its cote; the horse, dog, or cat finds its way home; the flock of birds migrates over a long distance; the nesting bird, insect or mammal locates its home spot, and each presumably because few or many of its senses are guiding it. It there fore appears that the careful study of the senses is essential to an understanding of many of the most interesting and puzzling things that an imals do.
The problem of animal orientation, especially distance orientation, has been attacked experi mentally by none so successfully as by the American psychologist, Watson, who has worked chiefly with white rats and terns. What Thorndike did for animal psychology in general by his pioneer study of cats and dogs, monkeys and chicks, Watson has done for the important problem of orientation.
Instinct: Hereditary Response.— Scientific progress has definitely settled the quarrel con cerning instinct and intelligence, for it has been proved that intelligence is possessed by other animals than man and that instinct is widely distributed in the animal kingdom. Human in telligence differs importantly from that of any other animal, but so does the intelligence of the chimpanzee from that of the cat.
Experiments have further demonstrated that instincts are not perfect and invariable from their first appearance, but instead become modi fied and perfected with use. They are inti mately related to habits, and in most animals form the basis for systems of habits which are developed through experience.
Studies in the heredity of mental traits and modes of response have been initiated by animal psychologists, and there are indications that the heretofore baffling problem of the origin and mode of development of instincts may shortly be solved. The modern experimental literature on instinct contrasts sharply with the older lit erature, for it presents definite facts in place of speculative imaginings, endless discussions, a priori definitions and theological disputes.