they manifest. And since a collective sentiment cannot express itself collectively except on the condition of observing a certain order permitting co-operation and movements in unison, these gestures and cries naturally tend to become rhythmic and regular; hence come songs and dances. But in taking a more regular form, they lose nothing of their natural violence; a regulated tumult remains tumult. The human voice is not sufficient for the task; it is reinforced by means of artificial processes: boomerangs are beaten against each other; bull-roarers are whirled. It is probable that these instruments, the use of which is so general in the Australian religious ceremonies, are used primarily to express in a more adequate fashion the agitation felt. But while they express it, they also strengthen it. This effervescence often reaches such a point that it causes unheard-of actions. The passions released are of such an impetuosity that they can be restrained by nothing. They are so far removed from their ordinary conditions of life, and they are so thoroughly conscious of it, that they feel that they must set themselves outside of and above their ordinary morals. The sexes unite contrarily to the rules governing sexual relations. Men exchange wives with each other. Sometimes even incestuous unions, which in normal times are thought abominable and are severely punished, are now contracted openly and with impunity.[702] If we add to all this that the ceremonies generally take place at night in a darkness pierced here and there by the light of fires, we can easily imagine what effect such scenes ought to produce on the minds of those who participate. They produce such a violent super-excitation of the whole physical and mental life that it cannot be supported very long: the actor taking the principal part finally falls exhausted on the ground.[703] To illustrate and make specific this necessarily schematic picture, let us describe certain scenes taken from Spencer and Gillen. One of the most important religious ceremonies among the Warramunga is the one concerning the snake Wollunqua. It consists in a series of ceremonies lasting through several days. On the fourth day comes the following scene.