Why doesn't the compiler see overloaded base class functions?

Why doesn't the compiler see overloaded base class functions? I cannot explain why the follwing is an error:

    #include <string>
    #include <iostream>
    using namespace std;

    class Base
    {
      public:
        virtual void Do( const string& )
        {
          cout << "Base::Do( string )" << endl;
        }
        virtual void Do( const char* const data,
                                  const long length )
        {
          cout << "Base::Do( char*, long )" << endl;
        }
    };

    class Derived : public Base
    {
      public:
        virtual void Do( const int s )
        {
          cout << "Base::Do( int )" << endl;
        }
    };

    int main(int argc, char* argv[])
    {
      Derived d;
      d.Do( 5 );
      d.Do( string() );
      d.Do( "foobar", 6 );
      return 0;
    }

Why doesn't the compiler "see" the base class versions of Do()? The only way I can get this to compile is to qualify the calls by adding "Base::", like so:

    d.Base::Do( string() );
    d.Base::Do( "foobar", 6 );

Can someone explain to me why this is the case? Thanks!