In school also, when the child learns to perform such an act as making the figure 2, the same changes take place. Here an impression must first proceed from the given copy to a sensory centre in the cortex. As yet, however, there is no vital connection established between the sensory centres and the motor centres which must direct the muscles in making the movement. As the movement is attempted, however, faint connections are set up between different centres. With each repetition the connection is made stronger, and the formation of the figure rendered less difficult. So long, however, as the connection is established within the cortex, the movement will not take place except under conscious direction. Ultimately, however, similar connections between sensory and motor neurons may be established in lower centres, whereupon the action will be performed as a reflex act, or without the intervention of a directing act of consciousness. This evidently takes place when a student, in working a problem, can form the figures, while his consciousness is fully occupied with the thought phases of the problem. Thus the neural condition of physical habit is the establishment of easy passages between sensory and motor nerves in centres lower than the cortex. CHAPTER XXI INSTINCT =Definition of Instinct.=--In a foregoing section, it was seen that our bodily movements divide into different classes according to their source, or origin. Among them were noted certain inherited spontaneous, but useful, complex movements which follow, in a more or less uniform way, definite types of stimuli presented to the organism. Such an inherited tendency on the part of an organism to react in an effective manner, but without any definite purpose in view, whenever a particular stimulus presents itself, is known as instinct, and the resulting action is described as an instinctive act. As an example of purely instinctive action may be taken the maternal instinct of insects whose larvae require live prey when they are born. To provide this the mother administers sufficient poison to a spider or a caterpillar to stupefy it, and then bears it to her nest. Placing the victim close to her eggs, she incloses the two together, thus providing food for her future offspring. This complex series of acts, so essential to the continuance of the species, and seemingly so full of purpose, is nevertheless conducted throughout without reference to past experience, and without any future end in view. Instinct may, therefore, be defined as the ability of an organism to react upon a particular situation so as to gain a desirable end, yet without any purpose in view or any previous training. =Characteristics of Instinct.=--An instinctive act, it may be noted, is distinguished by certain well marked characteristics: 1. The action is not brought about by experience or guided by intelligence, but is a direct reaction on the part of the organism to definite stimulation.