Singleton pattern in baseclass-subclass scenario

I am under the impression that, using Singleton pattern I can limit the number of instantiation of a class to one object. And keeping that in mind, have a look at the below code:

class Singleton(type):
_instances = {}
def __call__(cls, *args, **kwargs):
    if cls not in cls._instances:
        cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
    return cls._instances[cls]

class base1(object):
    __metaclass__ = Singleton

class base2(base1):
    pass

class base3(base1):
    pass

class base4(base2):
    pass        

obj1 = base4()
obj2 = base4()

print obj1 is obj2 #prints True

obj3 = base3()
obj4 = base3()

print obj3 is obj4 #prints True

print obj1 is obj3 #prints False

So the final print statement prints a False. What could be the best way to go ahead in achieving the goal, "Always return the same base1 object no matter which sub class instantiates it".