Page:Popular Science Monthly Volume 11.djvu/614

596 its work—that is, there is an adjustment between the internal relation of a certain sensation to the digestive process and the external relation between the presence of foreign matter in the stomach and food.

There is a relation between the sudden approach of a body toward the eye and danger to that organ, and to this relation the nerves and muscles are adjusted. There is a relation between a peculiar "cluck" and the presence of the mother-hen, and to this relation the chick responds. There grows up in the mind of the soldier a connection between the word "attention" and a particular attitude, and when he feels the sensation caused by the command, he at once, and unconsciously, performs the muscular movements necessary to assume that position; that is, there is an adjustment between the internal relation of a certain sensation to certain muscular movements and the external relation of a certain command to the necessity for assuming a given position. The sailor learns that there is a relation between certain signals and stormy weather, and to this relation his actions are adjusted. Finally, General Meyer finds that there is a relation between low barometric pressure in certain parts of the country and a liability to storms in other regions, and to this external relation he adjusts his actions.

The kind of external change to which an organism may respond of course varies greatly in different cases, both in constancy and complexity. The Venus's-flytrap is adjusted only to relations between objects on the surface of the leaf and in contact with it, but an animal with even rudimentary organs of sense may respond to changes which occur at a distance. Even the simple capacity to distinguish light from darkness is enough to enable an animal to perceive a distant body between it and the sun, and to adjust its actions accordingly.

As we ascend to creatures having more developed eyes, we find an increase in the sphere of surrounding space throughout which external relations can establish corresponding internal relations. A slight convexity of the epidermic layer lying over the sensitive tract first serves, by concentrating the rays, to render appreciable less marked variations in the quantity of light, and thus brings into view the same bodies at a greater distance, and smaller or less opaque bodies at the same distance. From this point upward, through the various types of aquatic creatures to the higher air-breathing creatures, we trace, under various forms and modifications, a complicated visual apparatus and a widening space through which the correspondence extends. It is needless to go into details. Hypotheses and illustrations aside, it is obvious that from the polyp, which does not stir till touched, up to the telescopic-eyed vulture or the far-sighted Bushman, one aspect of progressing life is the greater and greater remoteness at which visible relations in the environment produce adapted relations in the organism. The extension of the correspondence in space does not end