A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Nerve-cells and Fibres.—In the development of the individual animal certain cells from the primitive external layer or ectoblast of the embryo are set apart to preside over the relations of the creature to its environment. These cells are highly specialized, and while some of them are highly sensitive, others are adapted for carrying or transmitting the stimuli received by the sensitive cells, and still others have the function of receiving sense-impressions and of translating them into impulses of motion. The nerve-cells are receivers of impressions. These are gathered together in nerve-masses or ganglia, the largest of these being known as the brain, the ganglia in general being known as nerve-centres. The nerves are of two classes. The one class, called sensory nerves, extends from the skin or other organ of sensation to the nerve-centre. The nerves of the other class, motor nerves, carry impulses to motion.

The Brain, or Sensorium.—The brain or other nerve-centre sits in darkness, surrounded by a bony protecting box. To this main nerve-centre, or sensorium, come the nerves from all parts of the body that have sensation, the external skin as well as the special organs of sight, hearing, taste, and smell. With these come nerves bearing sensations of pain, temperature, muscular effort—all kinds of sensation which the brain can receive. These nerves are the sole sources of knowledge to any animal organism. Whatever idea its brain may contain must be built up through these nerve-impressions. The aggregate of these impressions constitute the world as the organism knows it. All sensation is related to action. If an organism is not to act, it cannot feel, and the intensity of its feeling is related to its power to act.

Reflex Action.—These impressions brought to the brain by the sensory nerves represent in some degree the facts in the animal's environment. They teach something as to its food or its safety. The power of locomotion is characteristic of animals. If they move, their actions must depend on the indications carried to the nerve-centre from the outside; if they feed on living organisms, they must seek their food; if, as in many cases, other living organisms prey on them, they must bestir themselves to escape. The impulse of hunger on the one hand and of fear on the other are elemental. The sensorium receives an impression that food exists in a certain direction. At once an impulse to motion is sent out from it to the muscles necessary to move the body in that direction. In the higher animals these movements are more rapid and more exact. This is because organs of sense, muscles, nerve-fibres, and the nerve-cells are all alike highly specialized. In the fish the sensation is slow, the muscular response sluggish, but the method remains the same. This is simple reflex action, an impulse from the environment carried to the brain and then unconsciously reflected back as motion. The impulse of fear is of the same nature. Reflex action is in general unconscious, but with animals, as with man, it shades by degrees into conscious action, and into volition or action "done on purpose."

Instinct.—Different animals show differences in method or degree of response to external influences. Fishes will pursue their prey, flee from a threatening motion, or disgorge sand or gravel swallowed with their food. Such peculiarities of different forms of life constitute the basis of instinct.

Instinct is automatic obedience to the demands of conditions external to the nervous system. As these conditions vary with each kind of animal, so must the demands vary, and from this arises the great variety actually seen in the instincts of different animals. As the demands of life become complex, so do the instincts. The greater the stress of environment, the more perfect the automatism, for impulses to safe action are necessarily adequate to the duty they have to perform. If the instinct were inadequate, the species would have become extinct. The fact that its individuals persist shows that they are provided with the instincts necessary to that end. Instinct differs from other allied forms of response to external condition in being hereditary, continuous from generation to generation. This sufficiently distinguishes it from reason, but the line between instinct and reason and other forms of reflex action cannot be sharply drawn.

It is not necessary to consider here the question of the origin of instincts. Some writers regard them as "inherited habits," while others, with apparent justice, doubt if mere habits or voluntary actions repeated till they become a "second nature" ever leave a trace upon heredity. Such investigators regard instinct as the natural survival of those methods of automatic response which were most useful to the life of the animal, the individual having less effective methods of reflex action perishing, leaving no posterity.

Classification of Instincts.—The instincts of fishes may be roughly classified as to their relation to the individual into egoistic and altruistic instincts.

Egoistic instincts are those which concern chiefly the individual animal itself. To this class belong the instincts of feeding, those of self-defense and of strife, the instincts of play, the climatic instincts, and environmental instincts, those which direct the animal's mode of life.

Altruistic instincts are those which relate to parenthood and those which are concerned with the mass of individuals of the same species. The latter may be called the social instincts. In the former class, the instincts of parenthood, may be included the instinct of courtship, reproduction, home-making, nest-building, and care for the young. Most of these are feebly developed among fishes.

The instincts of feeding are primitively simple, growing complex through complex conditions. The fish seizes its prey by direct motion, but the conditions of life modify this simple action to a very great degree.