Why does a function declaration with a const argument allow calling of a function with a non-const argument?

Take note of the following C++ code:

#include <iostream>
using std::cout;

int foo (const int);

int main ()
{
   cout << foo(3);
}

int foo (int a)
{
   a++;
   return a;
}

Notice that the prototype of foo() takes a const int and that the definition takes an int. This compile without any errors...

Why are there no compilation errors?