copy( quaternion ) {

		this._x = quaternion.x;
		this._y = quaternion.y;
		this._z = quaternion.z;
		this._w = quaternion.w;

		this._onChangeCallback();

		return this;

	}
Copies the values of the given quaternion to this instance.

@param {Quaternion} quaternion - The quaternion to copy.
@return {Quaternion} A reference to this quaternion.