简单列表显示的c#问题

我正在制作一个动物收容所应用程序，我必须在其中创建一只猫或一只狗，并通过创建它并将其添加到列表中。我有一个按钮应该显示列表中所有的动物，另一个按钮应该显示所有的狗。当我点击这两个按钮时，只会出现最后创建的动物。你能给我一些指导吗?

    namespace AnimalShelter
    {
    class AnimalShelter
    {
    private string Name;
    private int TelNumber;
    private List<Animal> AnimalList;
    public AnimalShelter(string name, int telnumber)
    {
        this.AnimalList = new List<Animal>();
        this.Name = name;
        this.TelNumber = telnumber;
    }
    public Animal FindAnimal(string id)
    {
        foreach (Animal anim in this.AnimalList)
        {
            if (id == anim.ChipRegistrationNumber)
            {
                return anim;
            }
        }
        return null;
    }

    public bool RemoveAnimal(string id)
    {
        if (id.Length <= 0)
            return false;
        Animal ao = this.FindAnimal(id);
        if (ao == null)
            return false;
        this.AnimalList.Remove(ao);
        return true;
    }
    public bool AddNewAnimal(Animal ao)
    {
        if (ao == null)
            return false;
        if (this.FindAnimal(ao.ChipRegistrationNumber) != null)
            return false;
        this.AnimalList.Add(ao);
        return true;
    }


    public List<Dog> GetAllDogs()
    {
       List<Dog> temp = new List<Dog>();
       foreach (Animal animal in this.AnimalList)
       {
           if (animal.GetType() == typeof(Dog))
               temp.Add((Dog)animal);
       }
       return temp;
    }
    public List<Cat> GetAllCats()
    {
        List<Cat> temp = new List<Cat>();
        foreach (Animal animal in this.AnimalList)
        {
            if (animal.GetType() == typeof(Cat))
                temp.Add((Cat)animal);
        }
        return temp;
    }
    public List<Animal> GetAllAnimals()
    {
        return AnimalList;
    }
    }
}

这是实际的表单，我在其中创建了猫或狗，并有显示所有动物或显示所有狗的按钮!

   private void AnimalCreation_Click_1(object sender, EventArgs e)
    {
        string name = tbname.Text;
        string number = tbregno.Text;
        DateTime date = this.DateBroughtIn.Value.Date;
        DateTime lastwalked = this.LastWalked.Value.Date;
        string badhabits = tbbadhabbits.Text;
        if (name.Length <= 0)
        {
            MessageBox.Show("A Name Must Be Given");
        }
        if (number.Length <= 0)
        {
            MessageBox.Show(" A number must be given ");
        }
        if (date > DateTime.Now)
        {
            MessageBox.Show("Invalid date, can not be added");
        }
        else
        {
            Animal a = null;
            if (dog.Checked)
            {
                a = new Dog(number, date, name, lastwalked);
                this.MyAnimalShelter.AddNewAnimal(a);
                MessageBox.Show("Dog Successfully added");
            }
            if (cat.Checked)
            {
                if (badhabits.Length <= 0)
                {
                    MessageBox.Show("Bad Habbits must be filled");
                }
                else
                {
                    a = new Cat(number, date, name, badhabits);
                    this.MyAnimalShelter.AddNewAnimal(a);
                    MessageBox.Show("Cat Successfully added");
                }
            }
        }
    }
    private void AllAnimalsShow_Click(object sender, EventArgs e)
    {
        foreach (Animal animal in MyAnimalShelter.GetAllAnimals())
        {
            listBox1.Items.Clear();
            listBox1.Items.Add(animal);
        }
    }
    private void AllDogsShow_Click_1(object sender, EventArgs e)
    {
        foreach (Animal animal in MyAnimalShelter.GetAllDogs())
        {
            listBox1.Items.Clear();
            listBox1.Items.Add(animal);
        }
    }