为什么在ajQuery.extend中添加方法就报错，没有这个方法？

ajQuery.extend({

merge:function(first,second){

var len = +second.length,

            j = 0,

            i = first.length;

            for ( ; j < len; j++ ) {

            first[ i++ ] = second[ j ];

            }

            first.length = i;

            return first;

},

b:function(){

alert('111');

}

})

$.merge is not a function