为什么这样的调用会调用到父类中的函数？

Question

#include
using namespace std;

class Ca{
public:
void do_down(void)
{
cout << "Ca down" << endl;
}
};

class Cb : public Ca
{
public:
void do_down(void)

{
cout << "Cb down" << endl;
}
};

class Cc : public Ca
{
public:
void do_down(void)

{
cout << "Cc down" << endl;
}
};

int main(void)
{
Ca *a1;

Cb b1;
a1 = &b1;
a1->do_down();

Cc c1;
a1 = &c1;
a1->do_down();

}
请教为什么编译运行后的结果是两个Ca down，而不是Cb down跟Cc down呢？