Get rid of XmlInclude when using XmlSerializer

How would you do this, if objects included inside one of the serialized objects was an abstract class? E.g.:

abstract class Base
{
    public int x;
}

class A : Base
{
    public int y;
}

class B : A
{
    public int z;
}

class SomeOtherClass
{
    public string s;
}

class SerializingClass
{
    public int w;
    public Base myB1;
    public Base myB2;
}

//AND you have an object
var sc = new SerializingClass{
   w = 10,
   myB1 = new B{ x = 1, y, 2, z = 3},
   myB2 = new A{ x = 10, y=20 }
};

How would you use your method to add just Base, A, B, and not SomeOtherClass without knowing the inner classes and derived classes held within your top level class which you request the serializer from? If you must know all of this, then I'd feel that this method is limited to shallow class structs and the XML attribute method works better (for this case).