Page:Insects - Their Ways and Means of Living.djvu/40

 what tells the creature that liberty is to be found above, and not horizontally or downward? Many people believe that these questions are not to be answered by human knowledge, but the scientist has faith in the ultimate solution of all problems, at least in terms of the elemental forces that control the activities of the universe.

We know that all the activities of animals depend upon the nervous system, within which a form of energy resides that is delicately responsive to external influences. Any kind of energy harnessed to a physical mechanism will produce results depending on the construction of the mechanism. So the effects of the nerve force within a living animal are determined by the physical structure of the animal. An instinctive action, then, is the expression of nerve energy working in a particular kind of machine. It would involve a digression too long to explain here the modern conception of the nature of instinct; it is sufficient to say that something in the surroundings encountered by the newly-hatched grasshopper, or some substance generated within it, sets its nerve energy into action, that the nerve energy working on a definite mechanism produces the motions of the insect, and that the mechanism is of such a nature that it works against the pull of gravity. Hence the creature, if normal and healthy in all respects, and if the obstacles are not too great, arrives at the surface of the ground as inevitably as a submerged cork comes to the surface of the water. Some readers will object that an idea like this destroys the romance of life, but whoever wants romance must go to the fiction writers; and even romance is not good fiction