c++: can vector<Base> contain objects of type Derived?

The title pretty much says it all. Basically, is it legal to do this:

class Base {
    //stuff
}

class Derived: public Base {
    //more stuff
}

vector<Base> foo;
Derived bar;
foo.push_back(bar);

Based on other posts I've seen, the following is okay, but I don't want to use pointers in this case because it's harder to make it thread safe.

vector<Base*> foo;
Derived* bar = new Derived;
foo.push_back(bar);