function viewList(genus){
  var btn = document.getElementById('btn'+genus).style;
  var obj = document.getElementById(genus).style;
  btn.background = (obj.display != 'block')?
                     'url(http://www.1023world.net/ypark/item/btn_list_on.gif) no-repeat left top':
                     'url(http://www.1023world.net/ypark/item/btn_list_off.gif) no-repeat left top';
  obj.display = (obj.display != 'block')? 'block': 'none';
}