How to use geomertryUtils merge in three js? - three.js

Question

I'm using old version of three js because some library compatible problem. There's no Geometry.merge function in it, all i could use is GeometryUtils.merge.
What i am trying to do is to have a mesh, and then merge other mesh into it(i mean merge their geometries) later in the animation loop.
The result is funny, the console.log indicates that the vertices have been added into the group geometry. I checked their positions, they seem right.
However, they seemly don't show up in the scene ( i can't see them!)
Notice that i called add() once before the animation function, and it works!
It just does not work in the animation loop. So weird!!!
var group = new THREE.Geometry();
var mmm = new THREE.Mesh(group, new THREE.MeshNormalMaterial());
scene.add(mmm);
var counter = 0;
function add(){
for(var i = counter + 1; i < counter + 2 ; i++){
var geo = new THREE.BoxGeometry(0.1, 0.1, 0.1);
var m = new THREE.Mesh(geo, material);
m.position.set(i, i, 0);
console.log(i)
group.verticesNeedUpdate = true;
THREE.GeometryUtils.merge(group, m);
}
counter ++;
console.log(group.vertices.length);
}
add();
var frameCount = 0;
function animate(){
requestAnimationFrame(animate);
if(frameCount !== 0 && frameCount % 60 === 0){
add();
}
frameCount++;
renderer.render( scene, camera );
}
animate();