its ventral side, connected with a similar pair of grouped nerve-cells above the anterior part of the digestive tract.  The ganglia of each segment exercise immediate supervision over the structures of their respective territory, while they pass on impulses to other ganglia so that movements involving many segments can be properly adjusted.  Everything an earthworm does is controlled by the cells grouped in these ganglia, or scattered along the intervening connecting cords.  We speak of its acts as instinctive, employing a term which seems to indicate a different kind of operation carried on by the nervous system, but a moment’s thought will show that an instinctive act is simply a complex group of reflex acts.  The physical basis and ultimate unit is a cell, and the functional unit is likewise a cell act; therefore the seeming difference proves to be one merely of degree and not of kind.  The greater complexity of the worm’s nervous system as compared with that of Hydra gives to the whole mechanism a plasticity that diverts the attention from the mechanical nature of the entire instinctive act and of its basic cell elements.

The instinct, like the elementary reflex, is determined by heredity.  Because a certain configuration of the cells and fibers making up a nervous system is inherited as well as the characters of the constituent elements themselves, a worm or an insect is enabled to act as it does.  A butterfly does not have to learn how to fly, for it flies instinctively.  When it emerges from its chrysalis with its complete adult series of wings and muscles, it has also the nervous mechanism by which these parts are mechanically controlled.  A ground-wasp deposits its eggs in a small burrow in which it places also a caterpillar or a grasshopper paralyzed by stinging, so that when the larva is hatched from an egg it finds an ample supply of fresh food provided by a complex series of its mother’s acts that seem to be directed by conscious maternal solicitude.  When the larva passes through the later stages of development and makes its way to the open air as a fully formed adult, it in its turn may go through the same course of action as its parent, but it is clear that it cannot have any remembrance of its mother’s work or any personal knowledge of the value of burying its own eggs in a chamber with a living prisoner to serve as food.  It was an egg when its parent did these things; as a parent itself it does not remain on watch to see how beneficial or fruitless its acts may be.  A mechanism produced by nature’s methods, the ground-wasp behaves as it is capable of working with its inherited structure and its inherited instinctive powers of cooerdination and sensation.