Xml-serilizing with interfaces

[grignard]

A list of interfaces which is implemented by an abstract klass with two subclasses that need to be serialised. I have no idea how to do this since interfaces can not be serialised as I understand it.
My code generates the error message:

Cannot serialize interface ConsoleApplication4.Creature_interface.


..allthough Im not trying to serilize the interfaces themselves.

Which is the right way to deal with a problem like this? Is there any stright
way to deal with this problem or is there some kind of workaround needed?

using System; 
using System.Collections.Generic; 
using System.Text; 
using UtilitiesLibrary; 
using System.Xml.Serialization; 

namespace ConsoleApplication4 
{ 
    class Program 
    { 
        static void Main(string[] args) 
        { 
            Cat cat1 = new Cat("Knut", 6); 
            Dog dog1 = new Dog("Bertil", 5); 

            List<Creature_interface> Animallist = new List<Creature_interface>(); 
            Animallist.Add(cat1); 
            Animallist.Add(dog1); 

            UniversalSerializer.XMLSerialize("filename.xml", Animallist); 
            Console.ReadLine(); 
             
        } 
    } 

    public interface Creature_interface 
    { 
        string Name { get;} 
        int Age { get; } 
    } 

    public abstract class Creature : Creature_interface 
    {    
        string _name; 
        int _age; 

        public Creature(string name, int age ) 
        { 
            _name = name; 
            _age = age; 
        } 

        public Creature() 
        { } 
         
        public string Name 
        { 
            get { return _name; } 
            set { _name = value; } 
        } 

        public int Age 
        { 
            get { return _age; } 
            set { _age = value; } 
        } 

        public abstract void Speak(); 
    } 

    [Serializable] 
    public class Dog:Creature 
    {    
        public Dog 
            (string _name, int _age): base (_name, _age) 
        { } 

        public Dog { } 

        public override void Speak() 
        { 
            Console.WriteLine("Bark Bark"); 
        } 
    } 

    [Serializable] 
    public class Cat:Creature 
    { 
        public Cat 
            (string _name, int _age): base (_name, _age) 
        { } 

        public Cat  { } 



        public override void Speak() 
        { 
            Console.WriteLine("Mjau Mjau"); 
        } 
    } 

}

using System;  
using System.Collections.Generic;  
using System.Text;  
using System.IO;  
using System.Xml.Serialization;  
using System.Runtime.Serialization.Formatters.Binary;  

namespace UtilitiesLibrary  
{     
    public class UniversalSerializer  
    {  

        public static void XMLSerialize<T>(string filename, T objectToSerialize)  
        {     
             XmlSerializer Serializer = new XmlSerializer(typeof(List<Creature_interface>), new Type[] {typeof(Dog), typeof(Cat)});

             Stream fStream = new FileStream(filename, FileMode.Create,  
             FileAccess.Write, FileShare.None);  

             Serializer.Serialize(fStream, objectToSerialize);  
             fStream.Close();  
        }  
//more methods...  
    }  
}

Thank you for any help.