Question on C++ Thread Class and inheritance/polymorphism with POSIX pthread

Hello to everybody,
I cannot understand a strange behaviour of the following C++ code.
I'm on debian linux with gcc 4.1.2 and I'm using POSIX pthread.
I want to create a class which encapsulates the logic of threads in
order to have a simple Thread virtual class which must be INHERITABLE,
that means, I can create a thread with custom code simply inheriting
Thread class and overloading void run() method.

This is the code that I written:

class Thread
{
public:
       Thread();//constructor
       virtual ~Thread();//distructor
       int start();//method which allow user to launch the thread
       virtual void run();virtual empty method run, the user will put
the
thread's code here
private:
       static void * gate(void *);//access point for pthread_create
       pthread_t _tid;//thread ID
};

Thread::Thread() : _tid(0)
{

}

int Thread::start()
{
       //create the thread
       return pthread_create(&_tid, NULL,gate,this);
}

Thread::~Thread()
{
       //wait for thread reconjunction
       if (_tid)
               pthread_join(_tid,NULL);
}

void Thread::run()
{
       cerr << "hello, I'm the base thread" << endl;//debug print
}

void * Thread::gate(void * p)
{
       //cast the void pointer
       ((Thread *)p)->run();
       return NULL;
}

My intention is to inherit Thread and redefine Thread::run(), that is
for example :

class MyThread : public Thread
{
public:
void run();
};

MyThread::run()
{
cerr << "hello, I'm the derived thread" << endl;//debug print
}

In the main funztion I put

MyThread mt;
mt.start();
....
....
....

Well,
I excpect to see "hello, I'm the derived thread" on the output,
instead I obtain is :

"hello, I'm the base thread"

-------------
WHY ????
-------------

I defined run as a virtual function so, even after a cast to void *
and then a cast to Thread * of the address of mt ( that is a MyThread
* ), I would excpect that the called method is decided not on the base
of the type of the pointer (Thread *) but on the type of the original
object (MyThread *) recorded in the v-table. This is the basics of the
polymorphism ....

The strange thing is that's happen only using threads. With the
following code instead everithing works correctly :

MyThread mt;
Thread *p=&mt;
p->run();

OUTPUT :
"hello, I'm the derived thread".

Please,
let me know what is the motivation of this strange behaviour.

Many thanks.

Alessandro