why can we assign base class object reference to derived class pointer?

[adapanaidu]

class Base
{
public:
Base(int ii = 0 , char cc = 65) : i(ii) , c(cc) {}
int i;
char c;
};

class Derived
{
public:
Derived(int ii = 0 , char cc = 65 , int id = 10 , char cd = 66) : Base(ii , cc) , i(id) , c(cd) {}
int i;
char c;
};

void main()
{
Base bo;
Derived *dp = &bo;
}